Add session name to statusline JSON input

Currently, the statusline script receives session_id (UUID) but not the human-readable session name set via /rename command.

Request: Add a session_name field to the JSON input passed to statusline scripts, so custom statuslines can display the renamed session name instead of just the project directory.

Example:

{
  "session_id": "2798ee40-af85-4da5-bdc3-e7d9c628d7f8",
  "session_name": "Transfer CLI Settings from MacBook",  // <-- NEW FIELD
  "transcript_path": "...",
  "cwd": "...",
  "model": { ... },
  ...
}

Use Case:
This would allow statuslines to show meaningful context like:

02:30 PM | Opus 4.5 | Transfer CLI Settings from MacBook | Ctx: 19% | $0.41

Session names provide more context than directory names, especially when working on multiple tasks in the same project.

Current Workaround:
Currently displaying project directory name, which doesn't distinguish between different sessions in the same project.