Add AGENTS.md and related links (CLAUDE.md, GEMINI.md) to allow code generation tools to interact with the openshift/operator-framework-catalogd repository.

The document may detail information useful to agents such as patterns, conventions and architectural decisions while avoiding overly specific implementation details.

Considerations:
The operator-framework-catalogd repo should mainly have changes made to the release-4.18 branch, as part of backports from the operator-framework-operator-controller repo's catalogd subproject.

Some sample AGENTS.md for reference:

• operator-framework-olm/operator-lifecycle-manager
• operator-framerwork-olm/operator-registry
• operator-framework-olm/tests-extension
• operator-framework/api
• openshift/kubernetes
• kubernetes/kubernetes
• agents.md official samples