[FEATURE] Auto-archive on PR close: add a delay ("archive N days after merge") instead of archiving immediately

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Auto-archive on PR close works well and I don't want to turn it off —
the trigger is right, a merged PR really does mean that session's work is
done. The issue is that it archives the instant the PR closes.

I work across a large number of repositories and switch between them
constantly, so the recent-session list is how I answer "what was I doing on
this project?" A merge is frequently not the end of my involvement:
follow-up review comments arrive, a small fix is needed, or I just want to
look at what I did yesterday. Right now those sessions are gone from the
list the moment the PR merges, and finding them again means going through
the archive.

The two settings available today are both wrong for this: leaving
auto-archive on loses recent context too early, and turning it off means
archiving every session by hand, one confirmation dialog at a time.

Proposed Solution

Add a delay to the existing trigger rather than a new mechanism:

• Auto-archive on PR close → archive N days after the PR is merged/closed
• N is user-set (I'd use 7). N = 0 is exactly today's behavior, so it can

ship as the default and break nothing.

• Ideally the timer is based on the session's last activity if that's later

than the merge, so a session I'm still working in doesn't age out.

This is a small change to a setting that already exists, and it makes
auto-archive usable for people who want cleanup without losing the
"recently finished" window.

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

Configuration and settings

Use Case Example

_No response_

Additional Context

Environment

Claude desktop app, macOS.