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/cli.md

@@ -95,7 +95,8 @@ Type `/` to see an autocomplete dropdown of all available commands.
 |---------|-------------|
 | `/tools` | List all available tools grouped by toolset |
 | `/toolsets` | List available toolsets with descriptions |
-| `/model [name]` | Show or change the current model |
+| `/model [provider:model]` | Show or change the current model (supports `provider:model` syntax) |
+| `/provider` | Show available providers with auth status |
 | `/config` | Show current configuration |
 | `/prompt [text]` | View/set/clear custom system prompt |
 | `/personality [name]` | Set a predefined personality |