fix: improve /model user feedback + update docs

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.

website/docs/user-guide/messaging/index.md

@@ -63,7 +63,8 @@ hermes gateway status       # Check service status
 | Command | Description |
 |---------|-------------|
 | `/new` or `/reset` | Start fresh conversation |
-| `/model [name]` | Show or change the model |
+| `/model [provider:model]` | Show or change the model (supports `provider:model` syntax) |
+| `/provider` | Show available providers with auth status |
 | `/personality [name]` | Set a personality |
 | `/retry` | Retry the last message |
 | `/undo` | Remove the last exchange |