Adds an interactive setup wizard that lets users configure their provider,
API key, base URL, and model without setting environment variables.
Configuration is persisted to ~/.claw/settings.json (with 0600 permissions).
New features:
- `claw setup` CLI subcommand runs the wizard from the terminal
- `/setup` slash command runs the wizard inside the REPL (hot-swaps model)
- Ctrl+P hotkey in the REPL triggers /setup for in-session provider swap
- Stored provider config used as fallback when env vars are absent
- Three-tier auth resolution: env var > .env file > stored config
- RuntimeProviderConfig struct and validation in settings schema
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
