feat(mcp): make sync opt-in and address review

Opt-in (mcpSyncEnabled, default OFF; routes 403 until on). Review fixes:
- codex TOML read/validated with smol-toml: CRLF, inline tables and
  command-less tables no longer yield a duplicate [mcp_servers.x]; the new
  text is re-parsed before writing
- null-prototype tables and own-key checks; unsafe names ignored at every level
- servers switched off in their own CLI (codex/opencode/antigravity) are not copied
- only CLIs that are installed or already have a config file take part
- files receiving env/headers are left 0600; symlinked configs are written through
- one apply at a time (409), unique tmp files cleaned on failure, failed status
- routes set real HTTP status codes; api-reference section; format type single-sourced

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

package.json

@@ -105,6 +105,7 @@
     "jpeg-js": "^0.4.4",
     "node-pty": "^1.1.0",
     "qrcode": "^1.5.4",
+    "smol-toml": "^1.9.0",
     "undici": "^6.28.0",
     "uuid": "^14.0.0",
     "web-push": "^3.6.7",