Allow `sessionTitle` in `SessionStart` hook output (currently only `UserPromptSubmit` supports it)

Summary

UserPromptSubmit hooks can set sessionTitle via hookSpecificOutput. SessionStart hooks cannot. Please add sessionTitle support to SessionStart hook output.

Use case

Auto-rename each session to the basename of the launch folder, so the session picker / tab title / agent panel reflects which project the session belongs to without any manual /title step.

Current workaround (suboptimal)

A UserPromptSubmit hook can emit:

{"hookSpecificOutput":{"hookEventName":"UserPromptSubmit","sessionTitle":"<folder>"}}

This works, but the title only appears after the first prompt is sent — not at session launch. Sessions sitting on the welcome screen, or sessions that are spawned and immediately backgrounded, never get titled.

Proposed behavior

Honor hookSpecificOutput.sessionTitle when emitted from a SessionStart hook:

{"hookSpecificOutput":{"hookEventName":"SessionStart","sessionTitle":"my-project"}}

…and apply it before the first user prompt.

Why not just default sessionTitle to cwd basename?

That'd also be welcome, but exposing it via the hook is more flexible — users can derive titles from git branch, worktree name, env var, etc.

Environment

• Claude Code on Windows 11 / PowerShell
• Affects all platforms (the hook contract is OS-agnostic)