fix(acp): request summarized thinking from Claude

[Polarisyjr]

Summary

Fixes #982.

Claude models that default thinking.display to "omitted" produce no usable thinking text in BenchFlow ACP trajectories.

This PR:

• passes agent-specific _meta through ACPClient.session_new()
• configures claude-agent-acp to request thinking.display="summarized"
• keeps the configuration registry-driven instead of adding a Claude-specific runtime branch
• ignores empty agent_thought_chunk and agent_message_chunk events so trajectory step counts only reflect actual content

Validation

Using the same task (lake-warming-attribution) before and after enabling summarized thinking:

Model	Configuration	Non-empty thought events	Thinking chars
claude-sonnet-5	default	0	0
claude-sonnet-5	summarized	2	2126 (partial; stopped after confirming the result)
Haiku 4.5	default	9	3532
Haiku 4.5	summarized	9	3559

Added tests for:

• forwarding _meta through session/new
• omitting _meta when no session metadata is configured
• Claude's summarized-thinking registry configuration
• filtering empty thought/message chunks