chore(hygiene): add node_modules/ and admin-ui dist/ to .gitignore (bf-8qyif)

Hygiene sweep (2026-07-11 corpus-audit follow-up). Adds missing .gitignore
entries for build dirs/caches:
- node_modules/  (gitignore-gap: examples/sdk-tests/package.json present
  but node_modules/ not ignored)
- crates/miroir-proxy/admin-ui/dist/  (build output for the proxy admin UI;
  entry lands before untracking the tracked dist/ artifacts so they don't
  reappear as untracked noise)

Co-Authored-By: Claude <noreply@anthropic.com>
Bead-Id: bf-8qyif

.gitignore

@@ -15,3 +15,5 @@ miroir-proxy-linux-amd64.sha256
 coverage/
 lcov.info
 *.rlib
+node_modules/
+crates/miroir-proxy/admin-ui/dist/