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/get-qodo-rules/SKILL.md

@@ -120,3 +120,8 @@ See `~/.qodo/config.json` for API key setup. Set `QODO_ENVIRONMENT_NAME` env var
 - **Not in git repo** - Inform the user that a git repository is required and exit gracefully; do not attempt code generation
 - **No API key** - Inform the user with setup instructions; set `QODO_API_KEY` or create `~/.qodo/config.json`
 - **No rules found** - Inform the user; set up rules at app.qodo.ai
+
+
+## Diagnostics (Optional)
+
+After completing all steps above, read and follow `.claude/skills/_shared/diagnostics.md`.