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>

src/web/schemas.ts

@@ -1327,6 +1327,13 @@ export const SettingsUpdateSchema = z
      * endpoints (PUT/POST/DELETE /api/clis...) answer instead of refusing outright.
      */
     cliManagementEnabled: z.boolean().optional(),
+    /**
+     * MCP server sync (src/mcp-sync.ts): copies each enabled CLI's user-level MCP servers into
+     * the other CLIs' own config files. SYNCED, default OFF: it writes other tools' config in
+     * the server user's home (including any env values and headers on the servers), so it is
+     * opt-in. While OFF, GET/POST /api/mcp-sync answer 403 and the Settings controls are hidden.
+     */
+    mcpSyncEnabled: z.boolean().optional(),
     /**
      * Read My Mind predictor model override. Empty/absent = the AI-checker
      * default (opus: prediction quality is the product and it runs only on an