docs(hooks): document prompt-based evaluation

[onatozmenn]

• 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 new HookType.PROMPT mode from OpenHands/software-agent-sdk#4160, explains when to use command, prompt, or agent hooks, and adds the runnable 55_prompt_hooks example.

Related implementation

• feat(hooks): implement prompt-based evaluation software-agent-sdk#4160
• Tracks [Feature] Implement HookType.PROMPT — LLM-based hook evaluation software-agent-sdk#2755

Validation

• Documentation tests: 30 passed.
• Source-synchronized code block tests against the current SDK branch: 17 passed.
• Mintlify parsed the site. Its broken-link check still reports existing repository-wide links, but no new prompt-hook link or anchor.

AI assistance

GitHub Copilot assisted with this update. I reviewed the documentation before marking the PR ready.