Persist per-project session color and name via settings.json

Problem

/color and /rename are session-scoped. There's no way to set a default color or session name per project — users have to re-run both commands every time they open a repo.

Proposal

Add two optional fields to settings.json (readable from .claude/settings.json so it can be per-project):

• sessionColor: string (one of the /color options, e.g. "orange")
• sessionName: string (applied the same way /rename applies it)

On session start, if set, apply them automatically. User can still override via /color and /rename during the session.

Use case

Multi-repo workflows where visually distinguishing sessions matters. Currently each repo looks the same in the tab bar / UI until the user manually renames + colors it.

Alternatives considered

• SessionStart hook — hooks run shell commands and can't invoke slash commands, so this doesn't work today.