refactor: split gsd-tools.cjs into 11 domain modules under bin/lib/

Break the 5324-line monolith into focused modules:
- core.cjs: shared utilities, constants, internal helpers
- frontmatter.cjs: YAML frontmatter parsing/serialization/CRUD
- state.cjs: STATE.md operations + progression engine
- phase.cjs: phase CRUD, query, and lifecycle
- roadmap.cjs: roadmap parsing and updates
- verify.cjs: verification suite + consistency/health validation
- config.cjs: config ensure/set/get
- template.cjs: template selection and fill
- milestone.cjs: milestone + requirements lifecycle
- commands.cjs: standalone utility commands
- init.cjs: compound init commands for workflow bootstrapping

gsd-tools.cjs is now a thin CLI router (~550 lines including
JSDoc) that imports from lib/ modules. All 81 tests pass.
Also updates package.json test script to point to tests/.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -43,6 +43,6 @@
   "scripts": {
     "build:hooks": "node scripts/build-hooks.js",
     "prepublishOnly": "npm run build:hooks",
-    "test": "node --test get-shit-done/bin/gsd-tools.test.js"
+    "test": "node --test tests/*.test.cjs"
   }
 }