Warn or confirm when a new session reuses the --name of another active session

What happens: Starting a session with claude -n <name> (or /rename) silently allows several concurrently active sessions on the same machine to share an identical name. No warning is shown.

Why it matters: Identically-named live sessions are indistinguishable in the session list and in any tooling that keys off the name; it is easy to act in the wrong window (different branch/model/PR). Real case: two simultaneously active sessions both named surefire-3345, one running on one model and one on another — only the status line told them apart.

Request: When starting or renaming into a name already held by another active session, show a clear warning, or prompt for confirmation, with a --force (or --allow-duplicate-name) escape hatch to opt in deliberately.

Related but different: #62478 (closed) covers auto-generated names colliding on the same branch; this is about user-set names among active sessions.

Environment: Claude Code 2.1.205, macOS (darwin 25.5).