feat(agents): implement task DAG executor with parallel execution (#413)

Implement the full Task DAG execution system for multi-agent workflows:

## TaskExecutor (cortex-agents/src/task/executor.rs)
- Parallel execution of independent tasks with configurable concurrency
- Respects DAG dependencies - tasks only start when dependencies complete
- File conflict detection to prevent parallel access to same files
- Progress tracking with completion percentage
- Cancellation support
- Configurable fail-fast or continue-on-failure modes
- Per-task and overall timeout support
- Automatic persistence of DAG state during execution
- Integration with both InMemoryDagStore and DagStore backends

## Session Hydration (cortex-agents/src/task/hydration.rs)
- SessionHydrator for restoring DAG state between CLI sessions
- Automatic reset of running tasks to pending on restore
- StaleTaskChecker for detecting when files were modified since task completion
- SessionRestoreConfig for configurable restoration behavior

## New Exports
- ExecutorConfig, ExecutorError, ExecutorResult, ExecutionProgress
- TaskExecutor, TaskExecutionResult, TaskStore trait
- SessionHydrator, SessionHydrationError, SessionHydrationResult
- StaleTaskChecker, StaleTaskInfo, SessionRestoreConfig
- execute_dag() convenience function

Includes 40 passing tests covering:
- Parallel execution behavior
- Dependency ordering
- Fail-fast and continue-on-failure modes
- Task timeouts
- File conflict detection
- Cancellation
- Progress tracking
- Persistence during execution

Co-authored-by: Droid Agent <droid@factory.ai>

Author: echobt

cortex-agents/src/lib.rs

@@ -167,8 +167,11 @@ pub use collab::{
 
 // Re-export task types
 pub use task::{
-    DagBuilder, DagError, DagHydrator, DagResult, DagStore, InMemoryDagStore, PersistenceError,
-    SerializedDag, Task, TaskDag, TaskId, TaskSpec, TaskStatus,
+    execute_dag, DagBuilder, DagError, DagHydrator, DagResult, DagStore, ExecutionProgress,
+    ExecutorConfig, ExecutorError, ExecutorResult, InMemoryDagStore, PersistenceError,
+    SerializedDag, SessionHydrationError, SessionHydrationResult, SessionHydrator,
+    SessionRestoreConfig, StaleTaskChecker, StaleTaskInfo, Task, TaskDag, TaskExecutionResult,
+    TaskExecutor, TaskId, TaskSpec, TaskStatus, TaskStore,
 };
 
 // Re-export routing types