fix: add config-only credential setup

[Terminal Chai (terminalchai)]

Adds a config-only CLI entry point for re-running provider, API key, model, and optional LangSmith setup without starting an agent run. This gives users a recovery path when saved credentials are expired, exhausted, or otherwise need to be replaced.\n\nFixes #23.

[PRATHAM KUMAR (rajpratham1)]

This is a focused, user-facing improvement that introduces a proper --config mode instead of overloading the initial setup flow. The behavior is consistent, the CLI UX is clearer, and the documentation is updated alongside the implementation.

What's good
✅ Introduces a dedicated --config command instead of piggybacking on normal startup.
✅ Prevents accidental agent execution after configuration.
✅ Rejects invalid usage like combining --config with other arguments.
✅ Handles non-interactive terminals with a clear error.
✅ Updates README and CLI usage documentation.
✅ forceReconfigure cleanly reuses the existing setup UI rather than duplicating logic.
Minor suggestion

The implementation now has several branches like:

if (forceReconfigure) {
return "provider";
}

inside helper functions (getInitialStep, getNextStepAfterProvider, getNextStepAfterApiKey). Since the reconfigure flow is becoming a distinct mode, you could consider centralizing this behavior in a small state machine or a dedicated flow function. It would reduce repeated forceReconfigure checks and make future setup changes easier to maintain.

[Colin Francis (colifran)]

This is going to be superseded by #374 which makes init idempotent so you can re-run to reconfigure things.