Make /rename auto-generated titles more verbose / customizable

Problem

/rename (with no args) auto-generates a session title that's too short to be useful. Common case: I ask Claude to investigate a teammate's PR by number/URL, run /rename, and the new title doesn't include the PR number — or any of the concrete entities I clearly referenced. So I end up renaming manually every time, which defeats the point of the auto-generator.

What I'd like

Any of these would help, in order of preference:

1. A more verbose default prompt — bias toward including concrete entities Claude already saw in the conversation: PR numbers, repo names, file paths, ticket IDs, the task verb (investigate / fix / migrate / etc.).
2. A renamePrompt setting in settings.json — let users override the prompt used to generate the title. Same shape as the existing language setting.
3. An env var like CLAUDE_CODE_RENAME_PROMPT — for users who script their config.
4. Allow user-defined ~/.claude/commands/rename.md to shadow the built-in. Today it does intercept (verified locally), but a custom command can only inject a prompt — it can't actually set the session title, so the override is functionally useless. If user commands could call back into /rename <name>, this would be solvable in userland.

Why it matters

Session lists become hard to scan when every conversation is titled "Investigating an issue" or "Reviewing a pull request". A few extra entity tokens (pr#1234, homepage-webflow, etc.) are the difference between a useful history and a wall of noise.

Workaround today

Pass the name explicitly: /rename pr#1234 investigate auth middleware. Works, but it's exactly the manual step the auto-generator is supposed to eliminate.