feat: parallel phase execution

New features:
- /gsd:execute-phase: run all plans in a phase with parallel background agents
- /gsd:status: monitor background agent status from parallel execution
- Parallelization-aware planning with depends_on and files_modified frontmatter
- Automatic dependency analysis and wave-based execution

Configuration options in .planning/config.json:
- max_concurrent_agents (default: 3)
- skip_checkpoints (default: true)
- min_plans_for_parallel (default: 2)

Inspired by PR #43 from @davesienkowski - thank you for the design work!

package.json

@@ -1,6 +1,6 @@
 {
   "name": "get-shit-done-cc",
-  "version": "1.3.34",
+  "version": "1.4.0",
   "description": "A meta-prompting, context engineering and spec-driven development system for Claude Code by TÂCHES.",
   "bin": {
     "get-shit-done-cc": "bin/install.js"