[Feature]: Document OpenHands repository boundaries in unified docs

[neubig]

Desired Behavior

The unified OpenHands documentation repository should explain the division of labor among the source repositories and products: Agent Canvas in OpenHands/OpenHands, the Python SDK and Agent Server in OpenHands/software-agent-sdk, the browser client in OpenHands/typescript-client, automation scheduling and dispatch in OpenHands/automation, and reusable skills/plugins/integrations in OpenHands/extensions.

Acceptance Criteria

• Add a repository-boundaries section to AGENTS.md.
• Add a repository-boundaries section to README.md or the canonical contributor/development documentation.
• Add the same ownership model to the docs repository custom code-review guide.
• Explain the dependency flow from SDK/Agent Server through OpenAPI and typescript-client to Agent Canvas.
• Explain that automation manages when work runs and the Agent Server/SDK executes conversations.
• Explain that reusable skills, plugins, automations, and integrations belong in extensions.
• State that PRs opened in the wrong repository should potentially be closed and moved to the owning repository.
• State that PRs must follow the applicable custom code-review guidance.
• Keep the guidance concise and avoid conflicting with the existing product architecture documentation.

Additional Context

Issue #686 covered broader stale product architecture and is closed; this issue is limited to the cross-repository contribution boundary.
This issue was created by an AI agent (OpenHands) on behalf of the user.