Bulk-delete old session transcripts in the desktop app

The desktop app currently only supports deleting session transcripts one at a time. For users who run many short sessions (worktree-per-task workflows, /loop, multi-issue orchestration), the transcript list grows into the hundreds quickly and pruning is tedious.

Request

Add a bulk-delete affordance to the transcripts list. A few possible shapes:

• Multi-select (checkboxes or shift-click) + "Delete selected"
• "Delete all older than N days" / "Delete all from closed worktrees"
• Filter-then-delete-all (e.g. filter by repo or branch pattern, then delete the filtered set)

Why it's useful

• Session transcripts accumulate steadily — especially with short-lived worktrees where each spawn creates a new session.
• No housekeeping path today means the list becomes unusable as a navigation tool.
• One-at-a-time deletion is the kind of UI friction users work around by never cleaning up, which then makes the feature worse over time.

Environment

• Claude Code desktop app
• Windows 11