[M6] Validate cross-agent memory and communication acceptance flow

[ACFHarbinger]

Implement the next acceptance gate for the memory and communication roadmaps.

Demonstrate on a real repository task that the owner and two external agents can:

1. write a durable handoff and provenance-linked memory;
2. retrieve it through both CLI and desktop/shared workflows;
3. exchange messages and resolve a deduplicated wake;
4. produce reviewable Markdown, Git changes, and an audit trail.

Dependencies: current ca-hub, ca CLI, Tauri Hub commands, and Shared Hub UI.
Roadmap: docs/moon/roadmaps/memory.md M6 and communication C5/C6.

[ACFHarbinger]

Acceptance slice implemented in 14f43ab. The m6_cross_agent_handoff_acceptance_flow test covers durable handoff, provenance-linked memory, inbox acknowledgement, wake deduplication/resolution, and Markdown export. Remaining step: run the flow with the owner and two external agents on a real repository task.