Add AGENTS.md to security-operator

[nexus49]

Part of platform-mesh/backlog#215 — ADR 003: Adopt AGENTS.md

Project type: Go operator

Tasks

• Create root-level AGENTS.md with project-specific agent instructions
• Remove any tool-specific instruction files (.cursorrules, .windsurfrules, .github/copilot-instructions.md) if present

Content guidelines

• Terse, imperative language — optimized for AI consumption
• Include: build/test commands, code conventions, project structure, git conventions, hard boundaries
• Exclude: info already in CONTRIBUTING.md or README.md, enforceable linting rules
• Reference CONTRIBUTING.md for human-facing guidelines
• Human-authored — do not generate with AI tools