feat(agent): action audit log + kill-switch + dry-run/shadow mode

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Parent epic: #768 (Phase 0).

Goal

Every autonomous action recorded (who/what/why/outcome), a global + per-repo pause-all switch, and a dry-run mode that logs what would have happened without executing.

Deliverables

• Action audit records (extend existing audit-event infra)
• Global + per-repo kill-switch honored by the action layer
• Dry-run/shadow mode for every action; feeds recommendation-outcome learning loop
• Revert-where-possible affordance

Context

existing audit events, src/services/recommendation-outcomes.ts, the preview-simulator pattern (settings-preview).

Milestone: M2.