feat(/usage): append account limits section in CLI and gateway

Wires the agent/account_usage module from the preceding commit into
/usage so users see provider-side quota/credit info alongside the
existing session token report.

CLI:
- `_show_usage` appends account lines under the token table. Fetch
  runs in a 1-worker ThreadPoolExecutor with a 10s timeout so a slow
  provider API can never hang the prompt.

Gateway:
- `_handle_usage_command` resolves provider from the live agent when
  available, else from the persisted billing_provider/billing_base_url
  on the SessionDB row, so /usage still returns account info between
  turns when no agent is resident. Fetch runs via asyncio.to_thread.
- Account section is appended to all three return branches: running
  agent, no-agent-with-history, and the new no-agent-no-history path
  (falls back to account-only output instead of "no data").

Tests:
- 2 new tests in tests/gateway/test_usage_command.py cover the live-
  agent account section and the persisted-billing fallback path.

Salvaged from PR #2486 by @kshitijk4poor. The original branch had
drifted ~2615 commits behind main and rewrote _show_usage wholesale,
which would have dropped the rate-limit and cached-agent blocks added
in PRs #6541 and #7038. This commit re-adds only the new behavior on
top of current main.

tests/gateway/test_usage_command.py

@@ -175,3 +175,79 @@ class TestUsageCachedAgent:
             result = await runner._handle_usage_command(event)
 
         assert "Cost: included" in result
+
+
+class TestUsageAccountSection:
+    """Account-limits section appended to /usage output (PR #2486)."""
+
+    @pytest.mark.asyncio
+    async def test_usage_command_includes_account_section(self, monkeypatch):
+        agent = _make_mock_agent(provider="openai-codex")
+        agent.base_url = "https://chatgpt.com/backend-api/codex"
+        agent.api_key = "unused"
+        runner = _make_runner(SK, cached_agent=agent)
+        event = MagicMock()
+
+        monkeypatch.setattr(
+            "gateway.run.fetch_account_usage",
+            lambda provider, base_url=None, api_key=None: object(),
+        )
+        monkeypatch.setattr(
+            "gateway.run.render_account_usage_lines",
+            lambda snapshot, markdown=False: [
+                "📈 **Account limits**",
+                "Provider: openai-codex (Pro)",
+                "Session: 85% remaining (15% used)",
+            ],
+        )
+        with patch("agent.rate_limit_tracker.format_rate_limit_compact", return_value="RPM: 50/60"), \
+             patch("agent.usage_pricing.estimate_usage_cost") as mock_cost:
+            mock_cost.return_value = MagicMock(amount_usd=None, status="included")
+            result = await runner._handle_usage_command(event)
+
+        assert "📊 **Session Token Usage**" in result
+        assert "📈 **Account limits**" in result
+        assert "Provider: openai-codex (Pro)" in result
+
+    @pytest.mark.asyncio
+    async def test_usage_command_uses_persisted_provider_when_agent_not_running(self, monkeypatch):
+        runner = _make_runner(SK)
+        runner._session_db = MagicMock()
+        runner._session_db.get_session.return_value = {
+            "billing_provider": "openai-codex",
+            "billing_base_url": "https://chatgpt.com/backend-api/codex",
+        }
+        session_entry = MagicMock()
+        session_entry.session_id = "sess-1"
+        runner.session_store.get_or_create_session.return_value = session_entry
+        runner.session_store.load_transcript.return_value = [
+            {"role": "user", "content": "earlier"},
+        ]
+
+        calls = {}
+
+        async def _fake_to_thread(fn, *args, **kwargs):
+            calls["args"] = args
+            calls["kwargs"] = kwargs
+            return fn(*args, **kwargs)
+
+        monkeypatch.setattr("gateway.run.asyncio.to_thread", _fake_to_thread)
+        monkeypatch.setattr(
+            "gateway.run.fetch_account_usage",
+            lambda provider, base_url=None, api_key=None: object(),
+        )
+        monkeypatch.setattr(
+            "gateway.run.render_account_usage_lines",
+            lambda snapshot, markdown=False: [
+                "📈 **Account limits**",
+                "Provider: openai-codex (Pro)",
+            ],
+        )
+
+        event = MagicMock()
+        result = await runner._handle_usage_command(event)
+
+        assert calls["args"] == ("openai-codex",)
+        assert calls["kwargs"]["base_url"] == "https://chatgpt.com/backend-api/codex"
+        assert "📊 **Session Info**" in result
+        assert "📈 **Account limits**" in result