docs: add kun-init 4-file documentation structure

[Soarecos]

Description

The objective of this PR is to introduce the standard kun-init 4-file documentation structure with strict ownership and no duplication across files:

• README.md — what the library is, installation, usage. Dev workflow removed and pointed at CONTRIBUTING.md.
• AGENTS.md — short agent-facing overview: what it is, domain, code layout, quality gates (real Composer scripts) and hard constraints.
• CONTRIBUTING.md — reconciled the existing file; now owns all development workflow (setup, quality gates) alongside the existing PR guidelines.
• CLAUDE.md — contains @AGENTS.md.

Issues

• KUNAI-8

Details

• Layout detected: library (type: library, PSR-4 Kununu\DataFixtures\, consumed as a Composer package). No k8s-env link added (not a deployed service).
• README ## Tests section moved into CONTRIBUTING.md to remove duplication; CI/SonarCloud badges kept.
• Quality gates reference the real Composer scripts; PHP version referenced via composer.json (require.php) rather than hardcoded.
• CLAUDE.md added normally (not listed in .gitignore).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud