feat: add versioned provider registry and doctor

[EmersonBraun]

Summary

• add a versioned registry for API providers and named local CLI providers
• keep grok, grok-cli, and opencode-cli distinct
• add offline doctor diagnostics with human and JSON output
• fail CI preflight on unknown local CLI versions
• update documentation and credential-safe offline tests

Verification

• npm run check
• npm pack --dry-run

Closes #38

[EmersonBraun]

Code review

No issues found after correction. Checked provider identity separation, API factory discovery, offline credential handling, CI version gating, Ollama HTTP preflight behavior, exit-code semantics, and documentation consistency. Local npm run check passes with 29 tests; required GitHub checks are green.