feat(execute): create @uncaged/workflow-execute + CLI migration

Phase 7: Engine + extract + workflow-as-agent merged into execute package.
All CLI imports migrated from @uncaged/workflow to specific packages.
105 CLI tests pass, 0 failures.

Changes:
- New @uncaged/workflow-execute package (engine/, extract/, workflow-as-agent)
- CLI src/ and __tests__/ rewritten to import from split packages
- bundle-validator updated to allow @uncaged/workflow-cas imports
- ensure-uncaged-workflow-symlink creates symlinks for all new packages

Ref: #143, closes #150

packages/workflow-execute/package.json

@@ -0,0 +1,25 @@
+{
+  "name": "@uncaged/workflow-execute",
+  "version": "0.2.0",
+  "type": "module",
+  "main": "src/index.ts",
+  "types": "src/index.ts",
+  "scripts": {
+    "test": "bun test"
+  },
+  "dependencies": {
+    "@uncaged/workflow-protocol": "workspace:*",
+    "@uncaged/workflow-runtime": "workspace:*",
+    "@uncaged/workflow-util": "workspace:*",
+    "@uncaged/workflow-cas": "workspace:*",
+    "@uncaged/workflow-reactor": "workspace:*",
+    "@uncaged/workflow-register": "workspace:*",
+    "yaml": "^2.7.1"
+  },
+  "peerDependencies": {
+    "zod": "^4.0.0"
+  },
+  "devDependencies": {
+    "zod": "^4.0.0"
+  }
+}