Implement send_message delivery for multi-session communication

[unforced]

Problem

The send_message MCP tool exists and validates inputs, but actual message delivery is not implemented. The code returns:

"Message delivery not yet implemented. Validation passed, but delivery requires SDK mid-stream message injection support."

Infrastructure for mid-stream message injection exists in claude_sdk.py (message_queue parameter) but is not wired to inter-session messaging.

Proposed

Either:

1. Finish the implementation — wire message_queue to send_message delivery
2. Disable the tool — remove it from available tools until ready, to avoid confusing the model

Context

The create_session tool works (spawns child sessions), but those sessions can't communicate back. This limits the multi-agent architecture to fire-and-forget patterns only.