User messaging improvements:
- Rejection: '(>_<) Error: not a valid model' instead of '(^_^) Warning: Error:'
- Rejection: shows 'Model unchanged' + tip about /model and /provider
- Session-only: explains 'this session only' with reason and 'will revert on restart'
- Saved: clear '(saved to config)' confirmation
Docs updated:
- cli-commands.md, cli.md, messaging/index.md: /model now shows
provider:model syntax, /provider command added to tables
Test fixes: deduplicated test names, assertions match new messages.
a23bcb81ce