docs: document rate-limit handling, token usage, /ws/system, and the workspace toolchain

Record the features that landed this cycle:
- CHANGELOG: provider rate-limit handling, token usage & cost analytics, the
  /ws/system operator stream; plus the fixes (agent gate toolchain, usage
  capture, panel endpoint shape + WS path, /public 500, provider pricing).
- CLAUDE.md: a WebSocket-streams section (incl. /ws/system + the
  websocket_bridge pattern), a Rate-limiting & usage subsystem note, and the
  'uv sync --extra dev' workspace-toolchain requirement.
- agent API reference: a System & realtime section (/api/system/rate-limits,
  /ws/system, per-resource WS streams).

docs/rag/tools/api-endpoints.md

@@ -85,6 +85,14 @@ Base URL: `http://{host}:{port}/api/v1`
 | GET | `/journals/me/stats` | My stats |
 | GET | `/journals/{agent}/entries` | Read team journal |
 
+## System & realtime
+
+| Method | Endpoint | Description |
+|--------|----------|-------------|
+| GET | `/api/system/rate-limits` | Active per-provider rate-limit state (`{ entries: [...] }`) |
+| WS | `/ws/system` | Operator stream — rate-limit lifecycle (`RATE_LIMIT_HIT` / `RATE_LIMIT_LIFTED`) |
+| WS | `/ws/agents/{id}`, `/ws/channels/{id}`, `/ws/sessions/{id}`, `/ws/notifications/{id}` | Per-resource live streams |
+
 ## Documentation
 
 | Method | Endpoint | Description |