docs(subagent): document loader invariants for file-based agents

[enyst]

Summary

Add openhands-sdk/openhands/sdk/subagent/AGENTS.md documenting the intended subagent loader design and invariants introduced by file-based agents (discovery locations, precedence/"first wins" resolution, and Markdown/YAML schema semantics).

Closes #2200.

Checklist

• If the PR is changing/adding functionality, are there tests to reflect this? (docs-only)
• If there is an example, have you run the example to make sure that it works? (n/a)
• If there are instructions on how to run the code, have you followed the instructions and made sure that it works? (n/a)
• If the feature is significant enough to require documentation, is there a PR open on the OpenHands/docs repository with the same branch name?
• Is the github CI passing?

Related user-facing docs PR: OpenHands/docs#358

---

Agent Server images for this PR

• GHCR package: https://github.com/OpenHands/agent-sdk/pkgs/container/agent-server

Variants & Base Images

Variant	Architectures	Base Image	Docs / Tags
java	amd64, arm64	eclipse-temurin:17-jdk	Link
python	amd64, arm64	nikolaik/python-nodejs:python3.12-nodejs22	Link
golang	amd64, arm64	golang:1.21-bookworm	Link

Pull (multi-arch manifest)

# Each variant is a multi-arch manifest supporting both amd64 and arm64
docker pull ghcr.io/openhands/agent-server:6c8e923-python

Run

docker run -it --rm \
  -p 8000:8000 \
  --name agent-server-6c8e923-python \
  ghcr.io/openhands/agent-server:6c8e923-python

All tags pushed for this build

ghcr.io/openhands/agent-server:6c8e923-golang-amd64
ghcr.io/openhands/agent-server:6c8e923-golang_tag_1.21-bookworm-amd64
ghcr.io/openhands/agent-server:6c8e923-golang-arm64
ghcr.io/openhands/agent-server:6c8e923-golang_tag_1.21-bookworm-arm64
ghcr.io/openhands/agent-server:6c8e923-java-amd64
ghcr.io/openhands/agent-server:6c8e923-eclipse-temurin_tag_17-jdk-amd64
ghcr.io/openhands/agent-server:6c8e923-java-arm64
ghcr.io/openhands/agent-server:6c8e923-eclipse-temurin_tag_17-jdk-arm64
ghcr.io/openhands/agent-server:6c8e923-python-amd64
ghcr.io/openhands/agent-server:6c8e923-nikolaik_s_python-nodejs_tag_python3.12-nodejs22-amd64
ghcr.io/openhands/agent-server:6c8e923-python-arm64
ghcr.io/openhands/agent-server:6c8e923-nikolaik_s_python-nodejs_tag_python3.12-nodejs22-arm64
ghcr.io/openhands/agent-server:6c8e923-golang
ghcr.io/openhands/agent-server:6c8e923-java
ghcr.io/openhands/agent-server:6c8e923-python

About Multi-Architecture Support

• Each variant tag (e.g., 6c8e923-python) is a multi-arch manifest supporting both amd64 and arm64
• Docker automatically pulls the correct architecture for your platform
• Individual architecture tags (e.g., 6c8e923-python-amd64) are also available if needed

[github-actions]

API breakage checks (Griffe)

Result: Passed

Action log

[github-actions]

Agent server REST API breakage checks (OpenAPI)

Result: Passed

Action log

[all-hands-bot]

🟢 Good taste - LGTM. Clear, well-structured documentation that solves a real problem: helping contributors understand agent discovery, precedence, and schema invariants. The logical flow (discovery → precedence → schema → debugging) makes it easy to find answers.