docs: explain per-subagent remote workspaces

[sohamb117]

Summary

Document per-subagent remote workspaces for Task and Delegate:

• Show the workspace factory API and registration through a custom TaskToolSet.
• Explain ownership, distinct sandboxes, local fallback, setup failure cleanup,
  resource lifetime, and in-session-only resume.
• Describe remote budget enforcement and compatible Agent Server requirements.

Companion SDK change: OpenHands/software-agent-sdk#4980.

Docs branch: feat/subagent-remote-workspaces, commit 4a905c81.
Changed page: sdk/guides/task-tool-set.mdx.

Validation

• git diff --check passed.
• Both added Python code blocks parsed successfully.
• Examples and ownership wording checked against the SDK implementation.
• SDK validation: 489 focused tests and four real-server cross-tests passed;
  a pre-final-rebase GPT-5-mini run passed with two isolated Docker children,
  in-session resume, result return, and cleanup.
• Mintlify rendering and broken-link checks have not been run locally.