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)

cli.py

@@ -5409,12 +5409,27 @@ class HermesCLI:
             print(f"  ❌ Compression failed: {e}")
 
     def _show_usage(self):
-        """Show cumulative token usage for the current session."""
+        """Show rate limits (if available) and session token usage."""
         if not self.agent:
             print("(._.) No active agent -- send a message first.")
             return
 
         agent = self.agent
+        calls = agent.session_api_calls
+
+        if calls == 0:
+            print("(._.) No API calls made yet in this session.")
+            return
+
+        # ── Rate limits (shown first when available) ────────────────
+        rl_state = agent.get_rate_limit_state()
+        if rl_state and rl_state.has_data:
+            from agent.rate_limit_tracker import format_rate_limit_display
+            print()
+            print(format_rate_limit_display(rl_state))
+            print()
+
+        # ── Session token usage ─────────────────────────────────────
         input_tokens = getattr(agent, "session_input_tokens", 0) or 0
         output_tokens = getattr(agent, "session_output_tokens", 0) or 0
         cache_read_tokens = getattr(agent, "session_cache_read_tokens", 0) or 0
@@ -5422,13 +5437,7 @@ class HermesCLI:
         prompt = agent.session_prompt_tokens
         completion = agent.session_completion_tokens
         total = agent.session_total_tokens
-        calls = agent.session_api_calls
 
-        if calls == 0:
-            print("(._.) No API calls made yet in this session.")
-            return
-
-        # Current context window state
         compressor = agent.context_compressor
         last_prompt = compressor.last_prompt_tokens
         ctx_len = compressor.context_length