Permanently delete archived sessions with merged pull requests after a retention period

[meganrogge]

Summary

Follow up to #334341 by permanently deleting archived agent sessions whose associated pull requests have merged after a configurable retention period. This would align the Agents Window lifecycle more closely with the GitHub app while preserving auto-archive as a reversible grace period before deletion.

Proposed behavior

• Only consider sessions that are already archived and whose associated pull requests are all merged.
• Require an additional retention period after archival before permanent deletion.
• Keep permanent deletion disabled by default.
• Revalidate pull request state, archived state, activity, and the current setting immediately before deletion.
• Use the existing session deletion lifecycle so persistence, provider callbacks, worktree cleanup, and client updates remain consistent.
• Provide an enterprise policy for administrators to disable or configure permanent deletion.

Design considerations

• Define behavior for local, remote, and externally sourced sessions.
• Decide whether associated worktrees and branches are deleted, retained, or independently configurable.
• Surface clear user messaging because deletion is irreversible.
• Handle partial failures without reporting a successful deletion.
• Ensure active or recently restored sessions can never be deleted from a stale candidate list.

Related: #334341

[meganrogge]

Permanent deletion is now included in #334341. The PR preserves a full archive grace period, only deletes sessions auto-archived by the feature, revalidates merged/session/configuration state inside the deletion critical section, and skips deletion while a preserved worktree remains. Closing this follow-up as superseded by the PR.