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

package.json

@@ -24,6 +24,7 @@
     "figlet": "^1.9.3",
     "gradient-string": "^3.0.0",
     "js-yaml": "^4.1.0",
+    "posthog-node": "^5.20.0",
     "zod": "^3.22.4",
     "zx": "^8.0.0"
   },