Fix missing osd-ui-framework dist CSS in production build (#12223)

After grunt was removed, packages/osd-ui-framework/package.json still
declares opensearchDashboards.build.intermediateBuildDirectory: 'target',
but its 'build' script ('grunt prodBuild') that used to populate target/
was deleted (scripts is now empty).

In the production build (@osd/pm buildProductionProjects) the per-project
flow is: deleteTarget() wipes target/, buildProject() is now a no-op (no
build script, no build targets), copyToBuild() then copies from the empty
target/. As a result only package.json is shipped and the committed
dist/kui_*.css files are dropped from node_modules/@osd/ui-framework.

At runtime core_app.ts serves /node_modules/@osd/ui-framework/dist/{path*}
from disk, so the missing kui_*.css returns a JSON 404 and the browser
blocks it on an X-Content-Type-Options: nosniff MIME-type mismatch,
briefly flashing the fatal-error (red) banner on initial load.

The KUI stylesheets are now pre-compiled and committed in dist/, so no
build step is required. Remove the stale intermediateBuildDirectory so the
package is shipped from its source root (which contains dist/). The build's
CleanExtraBuildFiles task strips source .scss/tests/docs from node_modules
while preserving .css.

Verified with a full distributable build: node_modules/@osd/ui-framework/dist/
now contains all six kui_*.css files (kui_v9_light.css included).

Signed-off-by: Justin Kim <jungkm@amazon.com>

Author: angle943

packages/osd-ui-framework/package.json

@@ -3,11 +3,6 @@
   "version": "1.0.0",
   "license": "Apache-2.0",
   "scripts": {},
-  "opensearchDashboards": {
-    "build": {
-      "intermediateBuildDirectory": "target"
-    }
-  },
   "dependencies": {
     "classnames": "^2.3.1",
     "lodash": "^4.18.0",