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>.

首先跟踪现有的 additionalContext 输出是如何针对 sessionStart 和 agentStop hook 进行解析和应用的。如果存在相关的 hook 输出测试和文档，请检查它们；完成的标准是 hook 可以提供 sessionTitle，并且 CLI 可以将其应用到 session，而无需 /rename。