Opt-in auto-archive for threads that stay settled

[voltcrash]

Problem

Auto-settle moves finished work out of the active list, but the Settled shelf keeps growing. A snapshot of my environment had 225 settled threads. Keeping older work off the sidebar requires recurring manual archiving.

Proposed behavior

Add Auto-archive settled threads alongside the existing auto-settle settings. It would be off by default. When enabled, it would archive threads that remain settled for a configurable number of days, with 30 days as the suggested initial value.

• Count the interval from the most recent settlement, for both manually and automatically settled threads.
• Keep archived conversations available in Settings → Archive so they can be restored.
• Cancel eligibility when a thread is un-settled or new activity resumes.
• Start a fresh interval when a settled thread is restored, so it stays visible for the full configured period.

Use environment defaults with project overrides, following the existing auto-settle settings. The controls would be available on web, desktop, and mobile, and the environment's server would apply the policy while clients are closed, including for remote connections.

Alternatives and tradeoffs

Automatically settled threads can include work someone still wants visible. Keeping this optional lets users choose whether to move that older work into the archive. Restricting eligibility to manually settled threads would be a possible smaller scope.

A manual bulk-archive action would also help clear the shelf, though it would still require periodic housekeeping. #6823 discusses that approach to the same growing-shelf problem.

Prototype

PR #13657 contains a prototype of these settings and the restore behavior. It was closed pending agreement on the direction and scope. These screenshots illustrate the proposed controls:

Auto-archive off (default)	Auto-archive enabled

Would opt-in auto-archiving be a useful addition? I'm happy to contribute it if maintainers want to pursue this direction, with eligibility and restore behavior adjusted to the agreed scope.