feat: add provider usage tracking

docs/concepts/usage-tracking.md

@@ -0,0 +1,27 @@
+---
+summary: "Usage tracking surfaces and credential requirements"
+read_when:
+  - You are wiring provider usage/quota surfaces
+  - You need to explain usage tracking behavior or auth requirements
+---
+# Usage tracking
+
+## What it is
+- Pulls provider usage/quota directly from their usage endpoints.
+- No estimated costs; only the provider-reported windows.
+
+## Where it shows up
+- `/status` in chats: adds a short “Usage” line (only if available).
+- CLI: `clawdbot status --usage` prints a full per-provider breakdown.
+- macOS menu bar: “Usage” section under Context (only if available).
+
+## Providers + credentials
+- **Anthropic (Claude)**: OAuth tokens in auth profiles.
+- **GitHub Copilot**: OAuth tokens in auth profiles.
+- **Gemini CLI**: OAuth tokens in auth profiles.
+- **Antigravity**: OAuth tokens in auth profiles.
+- **OpenAI Codex**: OAuth tokens in auth profiles (accountId used when present).
+- **z.ai**: API key via env/config/auth store.
+
+Usage is hidden if no matching OAuth/API credentials exist.
+