feat: add sentry-monitor extension and sentry-agent skill

Polls #bots-sentry Slack channel for new Sentry alerts (trigger layer),
then uses Sentry API for deep-dive on flagged issues (investigation layer).

Extension (sentry-monitor.ts):
- Polls Slack conversations.history on configurable interval (default 3min)
- Auto-resolves #bots-sentry channel ID
- Establishes baseline on first poll (no false alerts on existing messages)
- Parses alert severity, project, event counts, NEW/REGRESSION markers
- Sentry API: fetch issue details + full stack traces for triage
- State persisted via pi.appendEntry across restarts
- Uses correct pi extension API: registerTool, sendUserMessage, appendEntry

Skill (sentry-agent/SKILL.md):
- Triage guidelines: critical → immediate report, moderate → batch, low → silent
- Reports to control-agent via send_to_session with stack traces
- Tool reference and environment docs

Author: Hornet Agent