refactor(structure): relocate remaining legacy files out of main

dependency_whitelist.md

@@ -0,0 +1,13 @@
+# Dependency Whitelist
+
+Use this file only when a same-level or lateral dependency is unavoidable.
+
+Entry format:
+- requester: `<layer>/<module>`
+- target: `<layer>/<module>`
+- reason: short technical justification
+- owner: person/team
+- expiry: YYYY-MM-DD (must be reviewed before expiry)
+
+Current entries:
+- none