Expose session name in status line JSON payload

Feature Request

Summary: Add the session's friendly name (set via /rename) to the JSON payload passed to status line scripts.

Motivation: Currently the status line receives session_id (an internal UUID), but not the human-readable session name. Users who use /rename to organize their sessions would benefit from seeing the friendly name persistently in the status bar — especially when juggling multiple sessions.

Proposed change: Include a session_name field (or similar) in the status line JSON input, so scripts can display it. Example:

{ "session_id": "abc123...", "session_name": "my-feature-work", ... }

Use case: Display the current session name at the bottom of the CLI at all times, without needing to run /resume just to check which session you're in.