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/add-reactions/SKILL.md

@@ -115,3 +115,8 @@ Ask the agent to react to a message via the `react_to_message` MCP tool. Check y
 
 - Check IPC logs for `Unauthorized IPC reaction attempt blocked` — the agent can only react in its own group's chat
 - Verify WhatsApp is connected: check logs for connection status
+
+
+## Diagnostics (Optional)
+
+After completing all steps above, read and follow `.claude/skills/_shared/diagnostics.md`.