Add related sessions to the preview pane

[jongio]

Problem

When reviewing a session, the next useful session is often nearby in the same repo, branch, folder, or issue thread. Today users have to leave the preview, change filters, and search again to find that context.

Proposed solution

Add a Related sessions section to the preview pane. Rank nearby sessions using existing fields such as repository, branch, cwd, session refs, and last active time, then let the user jump to one directly.

Acceptance criteria

• The preview pane shows up to five related sessions with summary, repo or branch, and last active time.
• Ranking prefers shared refs, then same repo and branch, then same cwd, then recent activity.
• Selecting or clicking a related session moves focus to that session in the list.
• The section is hidden when no related sessions are found.
• Tests cover ranking order and navigation state changes.

Notes

Complexity: L · Priority: 4