feat(cortex-agents): implement multi-agent collaboration system

Implements AGENT_1 mission from ORCHESTRATE plan:

## Collaboration Module (collab/)
- spawn_agent: Create subagents with depth limit enforcement
- send_input: Inter-agent communication with interrupt support
- wait: Synchronize on multiple agents with configurable timeout
- close_agent: Graceful shutdown with RAII guards

## Task DAG Module (task/)
- TaskDag: Directed acyclic graph for task dependencies
- Topological sort for execution ordering
- Status propagation (completion unlocks dependents, failure skips)
- Persistence: Save/load DAG state to JSON
- Hydration: Create DAGs from task specifications

## Routing Module (routing/)
- DispatchMode: Parallel, Sequential, Background, Single
- decide_routing: Automatic dispatch mode selection
- File conflict detection for safe parallel execution
- Priority-based execution ordering

## Security Features
- MAX_THREAD_SPAWN_DEPTH = 1 (prevents infinite recursion)
- MAX_CONCURRENT_AGENTS = 10
- Timeout clamping (10s min, 5min max)
- AgentGuard for automatic cleanup

All 92 tests passing.

Author: factorydroid

Cargo.lock

@@ -13,3 +13,5 @@ tracing = "0.1"
 thiserror = "1"
 uuid = { version = "1", features = ["v4"] }
 regex = "1"
+futures = "0.3"
+chrono = { version = "0.4", features = ["serde"] }