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 named files under .github/rulesets/ and the referenced templates.hbs(github/rulesets) gist. Run the documented gh gist clone and just install commands to obtain the source definitions. Done means all four JSON files are present with the expected ruleset definitions and can be version-controlled in the repository.

Written by the indexing model from the issue text.