Add an outbox for external side effects

[pilipilisbot]

Context

Executor flow currently performs external effects such as GitHub reactions, OpenClaw dispatch, and completion notifications inline while processing a job. This works, but makes partial failure and retry semantics harder to audit.

Proposal

Introduce an outbox table for requested external effects with explicit idempotency keys, states, attempts, and last_error.

Acceptance criteria

• Reaction, dispatch, and notification effects can be recorded before execution.
• Retried effects are idempotent where the external API allows it.
• Job timeline distinguishes “decided”, “requested”, and “executed”.
• Monitor reports stuck outbox items.

Priority

Medium.