fix: use dedicated /api/oauth/usage endpoint for reliable plan data

Replaces the old approach (sending a real messages API request just to
read rate-limit headers) with the dedicated usage endpoint that Claude
Code CLI uses. Fixes intermittent "unknown" plan usage.

- GET /api/oauth/usage with Bearer auth + anthropic-beta header
- Auto-refresh expired OAuth tokens via refresh_token grant
- Try both keychain label formats (claude-code-credentials, Claude Code-credentials)
- Zero API quota consumption for usage checks
- Parse new JSON response format (five_hour/seven_day structure)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "openclaw-claude-proxy",
-  "version": "3.5.0",
+  "version": "3.5.1",
   "description": "OCP (Open Claude Proxy) — use your Claude Pro/Max subscription as an OpenAI-compatible API for any IDE. Works with Cline, OpenCode, Aider, Continue.dev, OpenClaw, and more.",
   "type": "module",
   "bin": {