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/cli-workflow/src/bundle-store.ts

@@ -1,7 +1,7 @@
 import { mkdir, readFile, writeFile } from "node:fs/promises";
 import { join } from "node:path";
 
-import { err, ok, type Result } from "@uncaged/workflow";
+import { err, ok, type Result } from "@uncaged/workflow-protocol";
 
 import { pathExists } from "./fs-utils.js";