docs(sdk): explain per-task LLM profile selection

[georgeglarson]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

Documents the per-call llm_profile option introduced by OpenHands/software-agent-sdk#4510.

The guide now covers model-selection precedence, unknown-profile errors, resume behavior, and the custom profile-store discovery limitation. It also corrects the documented default subagent type from default to general-purpose.

Validation: git diff --check passes. The docs test suite reaches 35 passes; its two remaining errors come from an existing pricing test that fetches a removed OpenHands source path and receives HTTP 404.