Provide guidance for AI assisted coding

Problem/Motivation

More and more contributions to this module are produced with AI coding assistants (Claude Code, Cursor, Copilot, …). Without project-specific guidance those agents ignore our conventions, and the cost of cleaning that up falls on the maintainers.

Proposed resolution

Implement the two cross-agent standards that solve exactly this:

• AGENTS.md — an open standard (stewarded by the Linux Foundation's Agentic AI Foundation) for project instructions. Plain Markdown, read natively by 20+ tools (Cursor, Codex, Copilot, Windsurf, …).
• SKILL.md — the cross-agent Agent Skills format for runnable procedures, loaded from .agents/skills/.

One committed source of truth, picked up as-is by any standard-compliant harness:

AGENTS.md                       # Always-on project conventions.
.agents/
  skills/                       # Runnable procedures, one folder per skill.
    run-checks/SKILL.md
    draft-issue/SKILL.md
    add-element-type/SKILL.md

Claude Code

Claude Code is lagging behind the rest of the AI ecosystem and does not support the standards yet (ref. anthropics/claude-code#6235); it reads only its own files CLAUDE.md and .claude/skills/. We can symlink the standard files.

CLAUDE.md      -> AGENTS.md
.claude/skills ->  ../.agents/skills

Remaining tasks

• Write AGENTS.md covering the core conventions: Drupal coding standards, the DDEV phpcs / phpstan / phpunit workflow, our test conventions, and the GraphQL schema-composition rules.
• Create .agents/skills/ with run-checks, draft-issue and add-element-type.
• Add and commit the Claude Code shim symlinks (CLAUDE.md, .claude/skills); update .gitignore to track them while ignoring per-machine agent state.