feat(coord): make GitHub projection retries idempotent

Follow-up from PR #375 review.

## Problem
A projection batch can partially succeed, then retry without advancing its cursor. Already-successful issue comments may be emitted again.

## Expected behavior
Projection retries are idempotent per logical outbound action, using an outbox or equivalent durable idempotency key.

## Verification
Inject a partial GitHub API failure after one successful comment, retry the same batch, and assert the successful comment exists exactly once while remaining actions complete.

Start with the review context in PR #375 and trace the projection batch retry path. Reproduce a partial GitHub API failure after one successful comment, retry the same batch, and verify that the successful comment exists exactly once while the remaining actions complete.

Written by the indexing model from the issue text.