feat: add opt-in diagnostics via PostHog

Per-event consent diagnostics that sends anonymous install/update/skill data
to PostHog. Conflict filenames are gated against upstream. Supports --dry-run
to show exact payload before sending, and "never ask again" opt-out via state.yaml.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.claude/skills/update-nanoclaw/SKILL.md

@@ -233,3 +233,8 @@ Tell the user:
 - Restart the service to apply changes:
   - If using launchd: `launchctl unload ~/Library/LaunchAgents/com.nanoclaw.plist && launchctl load ~/Library/LaunchAgents/com.nanoclaw.plist`
   - If running manually: restart `npm run dev`
+
+
+## Diagnostics (Optional)
+
+After completing all steps above, read and follow `.claude/skills/_shared/diagnostics.md`.