Define guidance for AI coding assistants

[stefanprodan]

Define the policy for using AI Agents when contributing to the Flux project.

Tasks:

• Add AI Coding Assistants Guidance to the contributing doc and website
• Add AGENTS.md to the following repos:
  • flux2
  • pkg
  • source-controller
  • source-watcher
  • kustomize-controller
  • notification-controller
  • helm-controller
  • image-reflector-controller
  • image-automation-controller
  • agent-skills (commit-assisted-by dev skill)