feat(agent): add async delegation continuation tool

[yukirora]

Summary

Add continue_delegation_async for non-blocking child Agent continuation while preserving the existing blocking behavior of continue_delegation. This enables main agent session talking to multiple child sessions non-blocking in multiple rounds for agent team mode.

Changes

• Add async continuation support for Claude and Pi runtimes.
• Extract shared continuation startup logic.
• Add Pi tool-call idempotency protection.
• Update the MCP catalog and collaboration Skill.
• Add runtime registration tests.

Validation

• Targeted tests: 4 passed
• Typecheck passed
• Main-process build passed
• Live test with two parallel async continuations succeeded