fix(agents): persist Anthropic refusal explanations

[solvemproblr]

Summary

• Claude HTTP 200 refusals (stop_reason=refusal) put the policy category and explanation on stop_details, but ClaudeAgent only copied stop_reason and left AgentStep.refusal empty.
• The trace UI already displays event.refusal; this copies Anthropic's explanation (or the category when explanation is missing) so Fable/Opus classifier refusals show the real reason instead of "The model refused this request."

Test plan

• uv run pytest hud/agents/tests/test_claude_agent.py -q (19 passed)
• Confirm a hosted Fable 5.1 refusal after the SDK pin is bumped shows the usage-policy text on the agent message

Made with Cursor

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