Add session name to statusline JSON input

Feature Request

The statusline script receives a JSON payload with session_id, but not the human-readable session name set via /rename.

Current behavior: The statusline JSON includes session_id (a machine-readable ID) but no session.name or session.title field.

Requested behavior: Add the session name (set via /rename) to the statusline JSON payload, e.g.:

{
  "session_id": "abc123",
  "session_name": "weekly-2026-02-09",
  ...
}

Use case: I name sessions with /rename to track weekly portfolio reviews (e.g., weekly-2026-02-09). Displaying the session name in the statusline would make it easy to identify which session I'm in at a glance, especially when running multiple sessions in parallel.

Workaround: None clean — session_id is opaque and transcript path parsing is fragile.