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 by reviewing the four listed paths under `.github/rulesets/` and the referenced `templates.hbs(github/rulesets)` gist. Run the provided `gh gist clone` and `just` installation commands to inspect the source. Done means all four JSON ruleset files are present in the repository and match the source definitions.

Written by the indexing model from the issue text.