docs: document external OCI registry agents as sub-agents

[dgageot]

Summary

Document the feature of using external OCI registry agents (e.g., agentcatalog/pirate) directly in sub_agents lists. This feature was demonstrated in examples/sub-agents-from-catalog.yaml but not covered in the docs.

Changes

• docs/concepts/multi-agent/ — Added "External Sub-Agents from Registries" section with examples covering auto-naming and name:reference syntax
• docs/concepts/distribution/ — Added "Using as Sub-Agents" section with a concise example and cross-link
• docs/configuration/agents/ — Updated sub_agents property description in schema and reference table
• docs/configuration/overview/ — Fixed validation rule to clarify external OCI references are pulled automatically
• docs/community/troubleshooting/ — Fixed contradicting statement about sub-agent references

Assisted-By: docker-agent

[docker-agent]

Assessment: 🟢 APPROVE

This documentation PR clearly and accurately documents the external OCI registry sub-agent feature. The changes are consistent across all documentation files, with proper cross-references and clear examples.

Summary:

• ✅ Consistent terminology and examples across 5 documentation files
• ✅ Accurate technical descriptions of OCI registry references
• ✅ Valid YAML syntax in all examples
• ✅ Proper cross-references between documentation pages
• ✅ No contradictions between sections

The documentation provides multiple examples showing different reference formats (agentcatalog/pirate, name:reference syntax, and fully qualified references with tags), making it easy for users to understand how to use this feature.

No bugs or issues found in the changed code.