refactor: rename push-review to Pushgate and update installation script

[dbrosio3]

Description

Introduces the M1 hook-to-runner boundary for Pushgate. The installed pre-push hook is now a small delegator into a managed pushgate command, with a protocol check and focused reinstall guidance when the runner is missing or incompatible.

The first runner entrypoint intentionally passes through pre-push after accepting Git hook args and draining stdin. Changed-file policy, deterministic command execution, skip controls, and AI policy stay with their owning follow-up issues.

Closes #4

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

• Added the standalone Node runner entrypoint with the hook-protocol probe and pass-through pre-push command.
• Replaced the legacy full hook body with a thin delegator that resolves $HOME/.pushgate/bin/pushgate, validates protocol 1, forwards Git args/stdin, and returns the runner exit code.
• Updated install.sh to install the managed runner, preserve pre-push backups, use rootstrap/ai-pushgate assets, and install .pushgate.yml templates instead of .push-review.yml.
• Reworked hook harness coverage around the runner boundary and added runner and installer tests with isolated Git repos and stubbed downloads.
• Updated README and contributing guidance to describe the current runner boundary and Node requirement without pulling later policy execution into this issue.

Testing

• pnpm test passes
• pnpm run check:shell passes with no output
• pnpm run lint:shell passes
• Manually tested the hook on a real repository
• Tested on macOS
• Tested on Linux

Covered by tests:

• Hook arg/stdin forwarding, runner exit-code propagation, missing/non-executable runner diagnostics, protocol mismatch diagnostics, and an installed-hook real push smoke path.
• Installer-managed runner placement, hook backup preservation, .pushgate.yml creation, config no-overwrite behavior, and no new .push-review.yml install destination.
• Runner protocol output, pass-through pre-push stdin draining, and unsupported command usage errors.

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 (not applicable; no new template)
• New template name added to install.sh usage comment (not applicable; no new template)

Screenshots / output

No screenshots. Terminal-facing behavior is covered by hook and installer diagnostics in the automated tests.