feat: Add ruleset JSON files to .github/rulesets/

## Description

Add GitHub ruleset JSON files from `templates.hbs(github/rulesets)` gist to enable version-controlled ruleset definitions.

## Files

- `.github/rulesets/main-branch-protection.json`
- `.github/rulesets/main-pr-reviews.json`
- `.github/rulesets/integration-branch-protection.json`
- `.github/rulesets/integration-pr-reviews.json`

## Source

Installed via:
```bash
gh gist clone ddb7525aa0b31d30415cbce2eb36bf59 gist-github-rulesets
INSTALL_DIR=. just --justfile gist-github-rulesets/github_rulesets.just install
```

Start with the four target paths under .github/rulesets/ and inspect the referenced gist installation command. Add the four JSON definitions from templates.hbs(github/rulesets), then verify that all listed files exist and match the source definitions.

Written by the indexing model from the issue text.