Feat: add POSIX-lite bootstrap foundation

Commands/Test/backend.just

@@ -0,0 +1,11 @@
+project_root := justfile_directory()
+backend_dir := project_root + "/Backend"
+
+# Run the full backend test suite.
+[default]
+all:
+	cd '{{backend_dir}}' && go test ./...
+
+# Run the isolated POSIX bootstrap smoke test.
+posix-bootstrap:
+	cd '{{backend_dir}}' && go test ./internal/bootstrap -run TestEnsureBootstrapPOSIXSkeletonInitializesEmptyRoot -count=1 -v

Commands/Test/mod.just

@@ -0,0 +1 @@
+mod backend