Release v0.3.0

- CLI parsing with `clap` derive (`src/cli.rs`)
- `/ai` and `/prompt` prefix detection to send prompts to the AI
- `Provider` trait for LLM provider abstraction (`src/provider.rs`)
- `AnthropicProvider` implementation with `reqwest::blocking`
- System prompt instructing the model to reply with fenced `bash` blocks or prose
- Response parsing: extract commands from fenced `bash`/`sh` blocks (`src/output.rs`)
- `OutputMode::Command` routes through safety layer → execution UI
- `OutputMode::Chat` prints prose directly to stdout
- Provider config section (`[providers.anthropic]`) with `api_key_env` and `base_url`
- Low-credit error detection with direct Anthropic billing link
- 20 new unit tests (cli, output, provider)