Improve AI development guidance (#21388)

Expand AGENTS.md with repository-specific guidance for design, lifecycle, OSGi, external data, security, testing, dependencies, formatting, and pull requests.
Add lightweight Claude and Gemini shims that delegate to AGENTS.md so the same instructions are reused across coding agents.

Signed-off-by: Wouter Born <github@maindrain.net>

Author: wborn