feat: offline-first dynamic agent registry and custom overrides

[4nrry]

This PR consolidates the dynamic agent registry implementation with a shift to an offline-first architecture.

Key Changes

• Offline-First Defaults: Agent rules (including Antigravity CLI support) are now embedded at compile-time via resources/default_agents.json.
• Local JSON Overrides: Removed background sync threads and SQLite storage for agent definitions. User overrides are now handled via ~/.opencherry/opencherry.json, with graceful fallback to embedded defaults on parse errors.
• Structural Simplification: Flattened AgentRule::exclude_path_contains from Option<Option<String>> to Option<String> and added #[serde(default)] to allow omission of optional fields in user configuration.

Verification

• cargo test --workspace passed.
• cargo clippy --workspace passed.
• Frontend Vitest suite (93 tests) and build passed.
• Tauri command signatures remain unchanged.

Credits

Thanks to @araujocode for the initial Antigravity CLI detection and forensic agent detection work that this PR builds upon.