Feature request: autocomplete/validation for /model command

Problem

When /model is given an invalid model name, the command silently fails with a "Model not found" message but provides no list of valid options. There's no way to discover valid model IDs without already knowing them.

Suggested improvement

• Add tab-completion for valid model IDs in the /model command
• Or print a list of valid model IDs when the command fails

Example

/model opus4.6
→ Model 'opus4.6' not found

It would be helpful if this instead printed:

Model 'opus4.6' not found. Valid models:
  claude-opus-4-6
  claude-sonnet-4-6
  claude-haiku-4-5-20251001

Filed on behalf of a user via Claude Code.