Add session tracking with reference counting for CLIProxy instances.
Multiple CCS sessions now safely share a single proxy on port 8317.
When any session exits, proxy only terminates if it was the last session.
Changes:
- Add session-tracker.ts: lock file based reference counting
- Modify cliproxy-executor.ts: reuse existing proxy, conditional cleanup
- Add 21 unit tests for session tracker functionality
Fixes#118
