feat(workspaces): introduce runtime execution boundary

Establish the pipe-mode vertical slice before migrating terminals, files, Git, and providers. Runtime selection is immutable per workspace, and external runtimes remain unavailable to path-based daemon surfaces until those capabilities move behind the boundary.

package.json

@@ -30,7 +30,8 @@
     "packages/relay",
     "packages/website",
     "packages/desktop",
-    "packages/cli"
+    "packages/cli",
+    "packages/docker-workspace-runtime"
   ],
   "scripts": {
     "dev": "npm run dev:server",