add spam detection to exploration: mute + skip storage for bot farms
when exploration identifies a spammer/bot farm, phi now:
- mutes them (server-side, survives restarts)
- stores one user-scoped marker instead of 5 detailed findings
- preserves structured rationale (mute_reason + evidence_uris)
adds /api/control/unmute endpoint for operator reversibility.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>