fix: move preseed.cfg to includes.installer for live-build

The preseed file needs to be in config/includes.installer/ for live-build
to embed it into the Debian installer. Previously it was in config/ which
doesn't get picked up by lb build.

- Moved config/preseed.cfg -> config/includes.installer/preseed.cfg
- Updated all test files to reference new path

💘 Generated with Crush

Assisted-by: GLM-4.7 via Crush <crush@charm.land>

tests/security/encryption_comprehensive_test.bats

@@ -2,11 +2,11 @@
 # Encryption configuration tests
 
 @test "LUKS2 encryption configured" {
-    grep -q "LUKS" /workspace/config/preseed.cfg
+    grep -q "LUKS" /workspace/config/includes.installer/preseed.cfg
 }
 
 @test "AES cipher configured" {
-    grep -qi "aes" /workspace/config/preseed.cfg
+    grep -qi "aes" /workspace/config/includes.installer/preseed.cfg
 }
 
 @test "encryption hooks exist" {