Feature request: expose session ID in UI for reliable cross-session messaging

When using cross-session messaging tools (e.g. an MCP-based "send_message to another session" tool), the target session must currently be located by fuzzy matching on title/branch/PR via a "list sessions" tool — there's no way to see or reference a session's ID directly in the UI.

This makes it easy to select the wrong session when titles are similar or ambiguous, especially when an agent resolves the target session ID automatically on the user's behalf without the user confirming which literal session it picked.

Request: surface the session ID (or a short unique identifier) somewhere in the UI (e.g. sidebar, session header) so users can unambiguously reference or copy it when they want to route a message to a specific session.