Define and implement completed-todo retention

[ladislas]

Summary

Define and implement completed-todo retention. Priority: P2.

Motivation

Todo garbage collection ages closed items from creation time. A month-old item closed now can be deleted at the next session startup; the focused reproduction demonstrated this behavior. Retention should explicitly define which lifecycle event starts the clock.

Reviewed source: extensions/todos/storage.ts. Revalidate against current main before implementation.

Acceptance Criteria

• Record the retention-policy decision, including the treatment of legacy closed todos; completion-based retention is the proposed default.
• Implement the agreed policy, including close/reopen/reclose transitions and a conservative migration that does not invent historical completion dates.
• Respect disabled GC and configured retention duration, and preserve open or concurrently reopened/edited items.
• Lifecycle tests cover old-created freshly closed items, genuinely expired closed items, reopening, legacy data, and disabled GC.
• Document retention and migration behavior for users.

Scope and verification

Resolve the lifecycle policy before implementation. Keep storage/locking changes proportional to the identified deletion risk.

Run affected regression coverage and required repository checks for any implementation; update relevant documentation/changelog under repository conventions. Record measurements and decisions durably in this issue or its linked PR.

Type

HITL — requires a policy or design review informed by the investigation before committing to the implementation approach. Investigation can proceed autonomously; publication follows existing authorization rules.

Parent

#466

Blocked by

None — investigation can start immediately; implement after the policy/design review described above.