Keep cross-issue task references in sync automatically #12

Outcome

Task-list references such as - [ ] #6 inside a parent issue update themselves when the referenced issue changes state, so tracking issues never go stale.

Evidence

Issue #6 was closed with a completion comment on 2026-08-22, but the - [ ] #6 checkbox in #9's delivery slice stayed unchecked until it was edited by hand. Every closed child leaves the same manual step behind.

Work

• Detect issue references (#N) in issue and comment bodies at create and update time.
• When a referenced issue opens or closes, re-render task-list checkboxes that point at it in the same repository.
• Record each automatic edit as an ordinary activity-log entry attributed to the system actor, never to a user.
• Keep the update bounded: only bodies containing a reference to the changed issue are rewritten.
• Cover authorization so private-repository references never leak into public issues.

Acceptance criteria

• Closing #N flips every - [ ] #N checkbox in the same repository without a human edit.
• Reopening #N restores the unchecked state.
• The automatic edit appears in the issue history with a distinct system actor.
• No cross-repository or private data is exposed by the rewrite.

Verification

Controller tests for close, reopen, and non-task references; authority tests for private repositories; mix precommit.

Dependencies

Builds on the Issues API core (#9) and fits naturally after #5.