ListAgents / cross-session messaging: expose user-assigned session titles

Title: ListAgents / cross-session messaging: expose user-assigned session titles, not just internal IDs

Description:

When running multiple Claude Code sessions at once (e.g. multiple terminal tabs/windows), each session gets an internal short ID (like zxc-a4, zxc-be) used by ListAgents and cross-session SendMessage. Separately, the UI lets me give each session/tab my own custom title (e.g. "P1后续任务", "帮忙干脏活的") for my own reference.

The problem: these two are completely disconnected. ListAgents only returns the internal ID, start time, and status — never my custom title. So when I want to tell Claude "send that to my '帮忙干脏活的' session," Claude has no way to resolve my title into the internal ID. It has to guess based on things like start time or ask the candidate session what it's doing.

This gets worse because the internal IDs aren't even stable — if a session's underlying process restarts or reconnects, it can get reassigned a completely different ID, while my custom title for that tab stays the same in the UI.

Requested feature: Have ListAgents (and/or the underlying session registry) also surface the user's custom session title alongside the internal ID, so Claude can directly match a session by the name the user actually knows it by, instead of doing indirect guesswork through timestamps or introspection messages.

Why it matters: This is especially useful for workflows where the user runs multiple Claude Code sessions in parallel for different purposes (e.g. one session scoped inside a repo, another kept clean outside it for handling a credential/token) and wants to route tasks between them by name rather than remembering/looking up opaque IDs each time.