docs: add AgentCore Registry integration steering doc

[iprivit]

Summary

Adds docs/AGENTCORE_REGISTRY.md, a steering doc describing how to register a deployed agent runtime in an existing Amazon Bedrock AgentCore Registry for discovery (A2A / MCP / agent skills / custom descriptor).

The doc is the source of truth for (re-)implementing the feature consistently across both IaC stacks. It covers:

• Architecture — one shared Python Lambda handler, two IaC frontends (CDK L2 construct via a CloudFormation Custom Resource; Terraform via a null_resource invoking the same index.py).
• Configuration surface — opt-in via registry_id, identical in shape across CDK config.yaml and Terraform tfvars.
• IaC → Lambda contract — the ResourceProperties payload, descriptor shaping per type, and best-effort approval.
• Descriptor content — the canonical object kept in sync across both stacks.
• CDK and Terraform implementation guides, IAM (the five bedrock-agentcore registry actions), a parity checklist, and testing/verification steps.

Scope

• Documentation only — this PR adds a single steering doc and no implementation.
• The feature is entirely opt-in: when no registry_id is configured, nothing is provisioned.
• References the post-L2-refactor structure (backend-construct.ts), rebased onto current main.

Test plan

• N/A (docs only). Reviewed for parity between the CDK and Terraform sections.