feat: implement Task DAG CLI system with parallel execution

This commit adds a comprehensive CLI interface for managing and executing
task DAGs (Directed Acyclic Graphs) with dependency resolution.

Features:
- DAG creation from YAML/JSON specification files
- Parallel execution with configurable concurrency limits
- Sequential execution mode with topological ordering
- Circular dependency detection during DAG validation
- Failure propagation with configurable modes (fail-fast, skip-dependents, continue)
- Task persistence and resume capability
- Multiple visualization formats (ASCII, Graphviz DOT, Mermaid)
- JSON, text, and compact output formats

CLI Commands:
- `dag create` - Create a new DAG from spec file
- `dag run` - Execute a DAG with parallel or sequential strategy
- `dag status` - Show DAG execution status
- `dag list` - List all stored DAGs
- `dag validate` - Validate DAG specification and detect cycles
- `dag graph` - Visualize DAG structure
- `dag delete` - Remove a stored DAG
- `dag resume` - Resume a partially executed DAG

Files added/modified:
- cortex-cli/src/dag_cmd.rs (new)
- cortex-cli/Cargo.toml (added cortex-agents and futures dependencies)
- cortex-cli/src/lib.rs (added dag_cmd module)
- cortex-cli/src/main.rs (added Dag command)

Author: factorydroid

Cargo.lock

@@ -38,6 +38,7 @@ cortex-mcp-server = { workspace = true }
 cortex-share = { path = "../cortex-share" }
 cortex-snapshot = { workspace = true }
 cortex-compact = { path = "../cortex-compact" }
+cortex-agents = { workspace = true }
 
 clap = { workspace = true }
 clap_complete = { workspace = true }
@@ -46,6 +47,7 @@ clap_mangen = { workspace = true }
 # For colored help output
 color-print = "0.3"
 tokio = { workspace = true, features = ["full"] }
+futures = { workspace = true }
 anyhow = { workspace = true }
 tracing = { workspace = true }
 tracing-subscriber = { workspace = true, features = ["env-filter"] }