Add sessionTitle field to hook output (like additionalContext) for programmatic session renaming

Currently, hook output only supports additionalContext injection (per docs). There's no way for a hook script to set/update the session title programmatically — the only path is a human typing /rename.

Claude Code supports this via hookSpecificOutput.sessionTitle in its SessionStart/UserPromptSubmit hook output — a deterministic script computes a title and the host applies it directly, no model involvement.

Requesting an equivalent sessionTitle field supported in Copilot CLI's sessionStart and/or agentStop hook output, so a script can rename a session based on info only known after the first prompt (e.g., detected worktree/ticket/PR), without requiring the user to copy-paste /rename <title>.

まず、sessionStart と agentStop フックに対して、既存の additionalContext 出力がどのように解析され適用されるかを追跡します。関連するフック出力のテストとドキュメントがあれば確認します。完了条件は、フックが sessionTitle を提供でき、CLI が /rename を必要とせずにそれをセッションに適用できることです。