Add OpenRouter as an MVP AI provider

[itsflaid]

Summary

• add arrow-key provider selection for Groq and OpenRouter during init
• default OpenRouter setup to openrouter/free while preserving any model explicitly chosen by the user
• route ask, analyze, config, and doctor through the configured provider
• add OpenRouter validation, completion, streaming, usage normalization, safe errors, and snapshot detection
• update the PRD, public docs, changelog, and personal testing notes

Verification

• 116/116 CLI unit tests passed
• TypeScript typecheck passed
• production CLI build passed
• built CLI help smoke test passed
• npm tarball pack passed and includes OpenRouter/provider artifacts
• git diff --check passed
• secret-pattern scan passed

Note

• full packed-package install E2E was not rerun locally because the desktop runtime did not provide an npm executable; tarball creation and existing package metadata tests passed

Closes #42