feat: capture provider rate limit headers and show in /usage (#6541)

Parse x-ratelimit-* headers from inference API responses (Nous Portal,
OpenRouter, OpenAI-compatible) and display them in the /usage command.

- New agent/rate_limit_tracker.py: parse 12 rate limit headers (RPM/RPH/
  TPM/TPH limits, remaining, reset timers), format as progress bars (CLI)
  or compact one-liner (gateway)
- Hook into streaming path in run_agent.py: stream.response.headers is
  available on the OpenAI SDK Stream object before chunks are consumed
- CLI /usage: appends rate limit section with progress bars + warnings
  when any bucket exceeds 80%
- Gateway /usage: appends compact rate limit summary
- 24 unit tests covering parsing, formatting, edge cases

Headers captured per response:
  x-ratelimit-{limit,remaining,reset}-{requests,tokens}{,-1h}

Example CLI display:
  Nous Rate Limits (captured just now):
    Requests/min [░░░░░░░░░░░░░░░░░░░░]  0.1%  1/800 used  (799 left, resets in 59s)
    Tokens/hr    [░░░░░░░░░░░░░░░░░░░░]  0.0%  49/336.0M   (336.0M left, resets in 52m)

agent/rate_limit_tracker.py

@@ -0,0 +1,242 @@
+"""Rate limit tracking for inference API responses.
+
+Captures x-ratelimit-* headers from provider responses and provides
+formatted display for the /usage slash command.  Currently supports
+the Nous Portal header format (also used by OpenRouter and OpenAI-compatible
+APIs that follow the same convention).
+
+Header schema (12 headers total):
+    x-ratelimit-limit-requests          RPM cap
+    x-ratelimit-limit-requests-1h       RPH cap
+    x-ratelimit-limit-tokens            TPM cap
+    x-ratelimit-limit-tokens-1h         TPH cap
+    x-ratelimit-remaining-requests      requests left in minute window
+    x-ratelimit-remaining-requests-1h   requests left in hour window
+    x-ratelimit-remaining-tokens        tokens left in minute window
+    x-ratelimit-remaining-tokens-1h     tokens left in hour window
+    x-ratelimit-reset-requests          seconds until minute request window resets
+    x-ratelimit-reset-requests-1h       seconds until hour request window resets
+    x-ratelimit-reset-tokens            seconds until minute token window resets
+    x-ratelimit-reset-tokens-1h         seconds until hour token window resets
+"""
+
+from __future__ import annotations
+
+import time
+from dataclasses import dataclass, field
+from typing import Any, Dict, Mapping, Optional
+
+
+@dataclass
+class RateLimitBucket:
+    """One rate-limit window (e.g. requests per minute)."""
+
+    limit: int = 0
+    remaining: int = 0
+    reset_seconds: float = 0.0
+    captured_at: float = 0.0  # time.time() when this was captured
+
+    @property
+    def used(self) -> int:
+        return max(0, self.limit - self.remaining)
+
+    @property
+    def usage_pct(self) -> float:
+        if self.limit <= 0:
+            return 0.0
+        return (self.used / self.limit) * 100.0
+
+    @property
+    def remaining_seconds_now(self) -> float:
+        """Estimated seconds remaining until reset, adjusted for elapsed time."""
+        elapsed = time.time() - self.captured_at
+        return max(0.0, self.reset_seconds - elapsed)
+
+
+@dataclass
+class RateLimitState:
+    """Full rate-limit state parsed from response headers."""
+
+    requests_min: RateLimitBucket = field(default_factory=RateLimitBucket)
+    requests_hour: RateLimitBucket = field(default_factory=RateLimitBucket)
+    tokens_min: RateLimitBucket = field(default_factory=RateLimitBucket)
+    tokens_hour: RateLimitBucket = field(default_factory=RateLimitBucket)
+    captured_at: float = 0.0  # when the headers were captured
+    provider: str = ""
+
+    @property
+    def has_data(self) -> bool:
+        return self.captured_at > 0
+
+    @property
+    def age_seconds(self) -> float:
+        if not self.has_data:
+            return float("inf")
+        return time.time() - self.captured_at
+
+
+def _safe_int(value: Any, default: int = 0) -> int:
+    try:
+        return int(float(value))
+    except (TypeError, ValueError):
+        return default
+
+
+def _safe_float(value: Any, default: float = 0.0) -> float:
+    try:
+        return float(value)
+    except (TypeError, ValueError):
+        return default
+
+
+def parse_rate_limit_headers(
+    headers: Mapping[str, str],
+    provider: str = "",
+) -> Optional[RateLimitState]:
+    """Parse x-ratelimit-* headers into a RateLimitState.
+
+    Returns None if no rate limit headers are present.
+    """
+    # Quick check: at least one rate limit header must exist
+    has_any = any(k.lower().startswith("x-ratelimit-") for k in headers)
+    if not has_any:
+        return None
+
+    now = time.time()
+
+    def _bucket(resource: str, suffix: str = "") -> RateLimitBucket:
+        # e.g. resource="requests", suffix="" -> per-minute
+        #      resource="tokens", suffix="-1h" -> per-hour
+        tag = f"{resource}{suffix}"
+        return RateLimitBucket(
+            limit=_safe_int(headers.get(f"x-ratelimit-limit-{tag}")),
+            remaining=_safe_int(headers.get(f"x-ratelimit-remaining-{tag}")),
+            reset_seconds=_safe_float(headers.get(f"x-ratelimit-reset-{tag}")),
+            captured_at=now,
+        )
+
+    return RateLimitState(
+        requests_min=_bucket("requests"),
+        requests_hour=_bucket("requests", "-1h"),
+        tokens_min=_bucket("tokens"),
+        tokens_hour=_bucket("tokens", "-1h"),
+        captured_at=now,
+        provider=provider,
+    )
+
+
+# ── Formatting ──────────────────────────────────────────────────────────
+
+
+def _fmt_count(n: int) -> str:
+    """Human-friendly number: 7999856 -> '8.0M', 33599 -> '33.6K', 799 -> '799'."""
+    if n >= 1_000_000:
+        return f"{n / 1_000_000:.1f}M"
+    if n >= 10_000:
+        return f"{n / 1_000:.1f}K"
+    if n >= 1_000:
+        return f"{n / 1_000:.1f}K"
+    return str(n)
+
+
+def _fmt_seconds(seconds: float) -> str:
+    """Seconds -> human-friendly duration: '58s', '2m 14s', '58m 57s', '1h 2m'."""
+    s = max(0, int(seconds))
+    if s < 60:
+        return f"{s}s"
+    if s < 3600:
+        m, sec = divmod(s, 60)
+        return f"{m}m {sec}s" if sec else f"{m}m"
+    h, remainder = divmod(s, 3600)
+    m = remainder // 60
+    return f"{h}h {m}m" if m else f"{h}h"
+
+
+def _bar(pct: float, width: int = 20) -> str:
+    """ASCII progress bar: [████████░░░░░░░░░░░░] 40%."""
+    filled = int(pct / 100.0 * width)
+    filled = max(0, min(width, filled))
+    empty = width - filled
+    return f"[{'█' * filled}{'░' * empty}]"
+
+
+def _bucket_line(label: str, bucket: RateLimitBucket, label_width: int = 14) -> str:
+    """Format one bucket as a single line."""
+    if bucket.limit <= 0:
+        return f"  {label:<{label_width}}  (no data)"
+
+    pct = bucket.usage_pct
+    used = _fmt_count(bucket.used)
+    limit = _fmt_count(bucket.limit)
+    remaining = _fmt_count(bucket.remaining)
+    reset = _fmt_seconds(bucket.remaining_seconds_now)
+
+    bar = _bar(pct)
+    return f"  {label:<{label_width}} {bar} {pct:5.1f}%  {used}/{limit} used  ({remaining} left, resets in {reset})"
+
+
+def format_rate_limit_display(state: RateLimitState) -> str:
+    """Format rate limit state for terminal/chat display."""
+    if not state.has_data:
+        return "No rate limit data yet — make an API request first."
+
+    age = state.age_seconds
+    if age < 5:
+        freshness = "just now"
+    elif age < 60:
+        freshness = f"{int(age)}s ago"
+    else:
+        freshness = f"{_fmt_seconds(age)} ago"
+
+    provider_label = state.provider.title() if state.provider else "Provider"
+
+    lines = [
+        f"{provider_label} Rate Limits (captured {freshness}):",
+        "",
+        _bucket_line("Requests/min", state.requests_min),
+        _bucket_line("Requests/hr", state.requests_hour),
+        "",
+        _bucket_line("Tokens/min", state.tokens_min),
+        _bucket_line("Tokens/hr", state.tokens_hour),
+    ]
+
+    # Add warnings if any bucket is getting hot
+    warnings = []
+    for label, bucket in [
+        ("requests/min", state.requests_min),
+        ("requests/hr", state.requests_hour),
+        ("tokens/min", state.tokens_min),
+        ("tokens/hr", state.tokens_hour),
+    ]:
+        if bucket.limit > 0 and bucket.usage_pct >= 80:
+            reset = _fmt_seconds(bucket.remaining_seconds_now)
+            warnings.append(f"  ⚠ {label} at {bucket.usage_pct:.0f}% — resets in {reset}")
+
+    if warnings:
+        lines.append("")
+        lines.extend(warnings)
+
+    return "\n".join(lines)
+
+
+def format_rate_limit_compact(state: RateLimitState) -> str:
+    """One-line compact summary for status bars / gateway messages."""
+    if not state.has_data:
+        return "No rate limit data."
+
+    rm = state.requests_min
+    tm = state.tokens_min
+    rh = state.requests_hour
+    th = state.tokens_hour
+
+    parts = []
+    if rm.limit > 0:
+        parts.append(f"RPM: {rm.remaining}/{rm.limit}")
+    if rh.limit > 0:
+        parts.append(f"RPH: {_fmt_count(rh.remaining)}/{_fmt_count(rh.limit)} (resets {_fmt_seconds(rh.remaining_seconds_now)})")
+    if tm.limit > 0:
+        parts.append(f"TPM: {_fmt_count(tm.remaining)}/{_fmt_count(tm.limit)}")
+    if th.limit > 0:
+        parts.append(f"TPH: {_fmt_count(th.remaining)}/{_fmt_count(th.limit)} (resets {_fmt_seconds(th.remaining_seconds_now)})")
+
+    return " | ".join(parts)