feat: restore source-declared filtering (requires common-repo >=0.28.0)

Source .common-repo.yaml includes only src/ and renames to root.
Consumers just need: repo: url + ref, no with: clause needed.

.common-repo.yaml

@@ -0,0 +1,7 @@
+# Source-level config: only expose src/ contents, renamed to root.
+- include:
+    - "src/**"
+    - "src/.*"
+    - "src/.*/**"
+- rename:
+    - "^src/(.*)$": "$1"