Add /models command to show all available models with status

[xmaks82]

Description

Add a `/models` CLI command that shows all supported models grouped by provider, with status indicators showing which ones are available (API key configured).

Expected output

```
Available Models:

Groq (GROQ_API_KEY: ✓)
llama-4-maverick Llama 4 Maverick (recommended)
llama-3.3-70b Llama 3.3 70B
qwen3-32b Qwen3 32B

SambaNova (SAMBANOVA_API_KEY: ✓)
samba-llama-70b Llama 3.3 70B (580 t/s!)
...

Anthropic (ANTHROPIC_API_KEY: ✗)
claude-sonnet Claude Sonnet 4 (paid)
...

Current: llama-3.3-70b (Groq)
```

Files to modify

• `src/agent/interfaces/cli.py` — add command handler
• `src/agent/clients/factory.py` — use get_available_models()