Let the geometry check borrow postcss without declaring it

The check compiles the app's stylesheet the way the app does, and reaches
postcss through the Tailwind plugin's own resolution rather than taking a
dependency on it. The dead-code gate reads the module name and asks for a
declaration; the borrow is deliberate, so the name is listed as one the gate
should leave alone.

knip.json

@@ -63,7 +63,8 @@
     "testcontainers",
     "tw-animate-css",
     "@napi-rs/canvas",
-    "@openai/codex-security"
+    "@openai/codex-security",
+    "postcss"
   ],
   "ignoreBinaries": ["docker", "docker-compose", "pg_isready", "tsx", "cosign"],
   "ignoreExportsUsedInFile": true,