scaffold: aports overlay, build script, CI workflow, README

Kernel aport forked from pmaports 7.1.2-r0, repointed at milos-linux
combined-stable with the tested FP6 config (+EFI_ZBOOT for packaging);
imsd + nftables aports from the pending pmaports submission. build.sh
carries TODO(validate) markers - not yet run end-to-end.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

aports/main/postmarketos-config-nftables/rules/60_usb.nft

@@ -0,0 +1,13 @@
+#!/usr/sbin/nft -f
+
+# Allow all traffic from usb-ethernet interfaces for debugging
+# and porting.
+
+table inet filter {
+	chain input {
+
+		# allow all from USB net
+		iifname "usb*" accept comment "accept USB net"
+
+	}
+}