Make /rename auto-generated session titles more descriptive (or configurable)

Summary

When /rename is invoked without arguments, the auto-generated session title is usually very short and generic (often just a few words that don't carry enough signal to distinguish sessions in /resume or the sessions list). The default output should either be richer by default, or be configurable.

claude --help has no relevant flag. The ~/.claude/settings.json schema has no key that controls this. There is no user-facing skill or hook for /rename; the prompt appears to be internal to the CLI.

Motivation

Multiple parallel sessions (often background jobs) across the same repo are common. In /resume or the sessions picker, short titles like "fix bug" or "update file" don't help identify the right one, which forces manual renaming nearly every time and defeats the purpose of the auto-generator.

Proposal

Either approach would help, and either can be tested if it ships behind a flag.

A. Make the default more descriptive. Aim for 8-15 words and include concrete nouns from the conversation: the file/feature touched, the symptom of a bug being fixed, or the user-visible goal. Example of the desired output:

• Current: "fix rename bug"
• Desired: "fix /rename losing session title after context compaction in VSCode"

B. Make the naming prompt configurable. Add a settings.json key, for example:

{
  "session": {
    "autoNamePrompt": "Summarize this session in 8-15 words. Include the primary file, feature, or symptom."
  }
}

This lets users with different workflows tune it without changing the binary, and keeps the default behavior unchanged for everyone else.