feat: add PostHog telemetry with persistent installation tracking

- Add telemetry module with PostHog integration and opt-out support
- Track workflow/agent lifecycle events (start, complete, fail, retry)
- Persist anonymous installation ID to ~/.shannon/telemetry-id
- Include hashed target hostname for unique target counting
- Mount host ~/.shannon in container for ID persistence across rebuilds

docker-compose.yml

@@ -29,6 +29,7 @@ services:
       - ./prompts:/app/prompts
       - ${TARGET_REPO:-.}:/target-repo
       - ${BENCHMARKS_BASE:-.}:/benchmarks
+      - ${HOME}/.shannon:/tmp/.shannon
     shm_size: 2gb
     ipc: host
     security_opt: