Feature Request: Expose custom session name in statusline JSON

Summary

When using /rename to set a custom session name (e.g., /rename T003), this friendly name is NOT available in the JSON passed to the statusline command.

Current Behavior

The statusline JSON only contains session_id which is a technical identifier (like abc123def...), not the user-defined display name.

Requested Behavior

Add a session_name or session_display_name field to the statusline JSON that contains the custom name set via /rename.

Use Case

Users who configure custom statuslines want to display meaningful session names (like T003, feature-auth, debug-api) instead of cryptic session IDs.

Example

Current JSON structure:

{
  "session_id": "abc123def456...",
  ...
}

Requested addition:

{
  "session_id": "abc123def456...",
  "session_name": "T003",
  ...
}

This would make the statusline much more useful for users who organize their work by session names.

Thank you! 🙏