feat: add provider-agnostic schema reviewers

[valkyriweb]

Summary

• Add provider-agnostic external reviewer support: codex, pi, command, and existing local smoke mode.
• Add shared CLI knobs: --reviewer-command, --reviewer-model, --reviewer-timeout-ms, --reviewer-concurrency, and repeatable --reviewer-arg.
• Keep --codex-* flags as deprecated aliases for backward compatibility.
• Add SQL CREATE TABLE extraction for .sql files and Markdown sql fences, including comment-aware splitting and source spans.
• Update README and bundled Schemator skill docs for Pi/provider-agnostic usage.

Verification

• npm run check — passes: typecheck, 197 Vitest tests, build.
• git diff --check — passes.
• Smoke-tested installed CLI with fake command reviewer and fake pi reviewer locally before commit.

Notes

A local reviewer pass initially caught SQL comment/span issues; those are covered by new regression tests. A second delegated reviewer run mis-rooted into another local repo, so it was discarded rather than treated as signal.