docs: add kun-init 4-file documentation structure

[Soarecos]

Description

The objective of this PR is to add the standard kun-init 4-file documentation structure to this repository, giving humans and coding agents a consistent entrypoint.

The four files have strict, non-overlapping ownership:

• README.md — what the bundle is, installation, and usage. Now links to the contributing guide instead of duplicating dev setup.
• AGENTS.md — short agent brief: purpose, domain, code layout, quality gates, and hard constraints.
• CONTRIBUTING.md — reconciled to own all dev workflow (local setup, quality gates, running tests, PR conventions).
• CLAUDE.md — points to @AGENTS.md.

Issues

• KUNAI-8

Details

• Created AGENTS.md (short-form) and CLAUDE.md (@AGENTS.md).
• Reconciled CONTRIBUTING.md: it now owns the full development workflow. Fixed stale references — the coding-standards package kununu-scripts is now kununu/code-tools (the actual dev dependency), and "Master is the stable branch" is now main. Migrated the local-setup / test instructions here from the README.
• Trimmed the duplicated "Testing the bundle" dev-setup section from README.md, renamed the section to ## Contributing, and fixed a typo in the usage links (FixtureAwareTextCase → FixturesAwareTestCase). Badges left untouched.
• Documentation only; no source or config changes. Quality gates are referenced by name and point to composer.json scripts rather than being hardcoded.

Noted, not changed (conservative docs-only scope)

• docs/FixturesTypes/*.md link to kununu/data-fixtures via blob/master. These are links into a separate third-party repository, so they were left as-is rather than retargeted.

[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