feat: add pre-commit review hook - #20

Summary

• add a managed Node hook for the pre-commit framework
• keep model-backed review manual by default, with an explicit pre-push option
• document provider, cost, latency, credential, and diff-boundary expectations
• include the hook manifest in the npm release payload

Why

Teams already using pre-commit should be able to invoke the same provider-neutral, adversarial review workflow without maintaining a custom wrapper. A full model-backed review is materially slower and more expensive than a formatter, so the hook does not silently run on every commit.

The hook passes no positional filenames because the CLI reviews a repository diff, explicit paths, a pull request, or stdin. The documentation also makes clear that a pre-commit invocation does not mean only staged files are reviewed.

Validation

• npm run check — 16 tests passed, including the new documentation contract
• Doc Bridge freshness and Documentation Standard v1 passed
• README Standard v1 passed
• npm pack --dry-run includes .pre-commit-hooks.yaml
• pre-commit validate-manifest .pre-commit-hooks.yaml
• git diff --check

This PR does not change CLI semantics, provider behavior, dependencies, the GitHub Action contract, or the package version.