Auto-archive on PR close archives long-lived "hub" sessions prematurely — scope it to PR ownership

Summary

The "Auto-archive on PR close" setting archives sessions that aren't finished units of work — specifically long-lived sessions whose role is to merge other sessions' PRs. When one of those other PRs closes, the still-active session is archived out from under the user.

Repro

1. Enable Settings → Auto-archive on PR close.
2. Keep one long-lived session open as a "hub" — you use it to review and merge other sessions' PRs (e.g. you run gh pr merge <N> there, or use a tool/skill that merges another session's PR).
3. Have a second, separate session that owns a feature branch + PR. Merge that PR from the hub session.
4. Observed: the hub session gets archived when the other session's PR closes — even though it has unfinished work and never owned that PR.
5. Expected: only the session that owns the merged PR (the one whose worktree branch is that PR's head branch) should auto-archive.

Suspected root cause

The trigger appears keyed on association ("a PR this session interacted with closed") rather than ownership ("this session's own branch PR merged").

Proposed fix — ownership-scoped trigger

Auto-archive a session only when ALL of these hold:

1. the session's working directory is a dedicated git worktree (not the main checkout / repo root), and
2. the merged PR's head branch == that worktree's checked-out branch (the session owns the PR; it didn't merely run the merge on someone else's), and
3. the session has no other open PR whose head branch it owns.

Sessions running from the repo root (which own no feature branch) would then never auto-archive — the desired behavior for hub / coordinator / inbox sessions.

Secondary request (optional)

Scheduled / recurring agents spawn one retained session per run; these pile up with no owning PR, so the ownership rule above won't reach them. A "keep the latest N runs per scheduled task, archive older" retention option would clear them separately.

Note for any programmatic workaround

The archive_session MCP tool is unavailable in unsupervised / headless mode, so this can't be worked around by a scheduled agent — the fix needs to live in the in-app auto-archive feature itself.