Feature: Hook output field to set session title (setSessionTitle)

Summary

Add a setSessionTitle (or similar) JSON output field for hooks — specifically SessionStart — so users can programmatically name their conversations.

Use Case

As a power user running multiple concurrent Claude Code terminal sessions, I want to name each session at startup so I can identify them later in the session picker. Currently:

• Session titles are auto-generated from the first message
• There's no way to override or set a title programmatically
• No hook output field exists for session metadata

Proposed Behavior

A SessionStart hook could return JSON like:

{
  "sessionTitle": "Web UI Reorganization"
}

Or combined with an elicitation/prompt to ask the user:

{
  "additionalContext": "Please name this session",
  "sessionTitle": "<user-provided value>"
}

Why Hooks?

The hook infrastructure already supports SessionStart events and JSON output with fields like additionalContext and decision. Adding a metadata field like sessionTitle would be a natural extension.

Alternatives Considered

• Manual rename command (e.g. /rename) — would also work but hooks allow automation
• Branch name as proxy — works for git repos but not general purpose