feat: add continue prompt hook

[frostming]

Summary

• add a first-result continue_prompt(tape, state) hook for agent-loop continuation prompts
• support both synchronous and asynchronous hook implementations and pass the updated StreamState
• preserve the builtin continuation prompt behavior, including tape context, and document the hook in English and Chinese

Verification

• uv run ruff check .
• uv run mypy src
• uv run pytest -q (278 passed)
• make docs-test (blocked locally before content compilation because astro-mermaid is missing from website/node_modules)

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	bub	a934f0c	Commit Preview URL Branch Preview URL	Aug 14 2026, 05:31 AM