Issue 1 pushgate v2 contract

[dbrosio3]

Description

Defines the ai-pushgate README contract for local pre-push checks and CI/PR enforcement before the runtime work moves over. The README now captures the expected install path, configuration shape, normal git push integration, local AI blocking defaults, skip controls, and privacy defaults.

Related: rootstrap#1

Type of change

• fix: — Bug fix (patch version bump)
• feat: — New feature (minor version bump)
• feat!: / fix!: — Breaking change (major version bump)
• docs: — Documentation only (no version bump)
• chore: — Maintenance or config (no version bump)
• refactor: — Code restructure, no behaviour change (no version bump)

Changes made

• README.md: replace the old push-review README framing with the ai-pushgate product contract, defaults, flow diagram, and config example.
• README.md: document installation, Git hook integration, skip controls, and AI privacy expectations for the new workflow.

Testing

• bash -n hook/pre-push passes with no output
• bash -n install.sh passes with no output
• Manually tested the hook on a real repository
• Tested on macOS
• Tested on Linux

Validation notes:

• GitHub Actions Validate shell scripts check passed for this PR.
• Template YAML validation was run while preparing the README contract update.

Checklist

• Commit messages follow Conventional Commits
• No eval, heredoc variable expansion, or unquoted variable interpolation in shell scripts
• File lists passed as arrays, never as interpolated strings
• VERSION, CHANGELOG.md, .release-please-manifest.json, and release-please-config.json were not manually edited — these are managed by release-please
• New template added to the table in README.md (if applicable)
• New template name added to install.sh usage comment (if applicable)

Screenshots / output

N/A. This PR updates README documentation only.