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/system/fde_test.bats

@@ -64,8 +64,8 @@
 
 # Test: Verify preseed has crypto partitioning
 @test "Preseed has crypto configuration" {
-    [ -f "config/preseed.cfg" ]
-    grep -q "crypto\|Crypto\|encrypted\|luks" config/preseed.cfg || true
+    [ -f "config/includes.installer/preseed.cfg" ]
+    grep -q "crypto\|Crypto\|encrypted\|luks" config/includes.installer/preseed.cfg || true
 }
 
 # Test: Verify encryption README is created

tests/system/secureboot_test.bats

@@ -39,8 +39,8 @@
 
 # Test: Verify preseed has UEFI/GPT configuration
 @test "Preseed uses GPT partitioning for UEFI compatibility" {
-    [ -f "config/preseed.cfg" ]
-    grep -q "gpt\|GPT" config/preseed.cfg || grep -q "efi\|EFI" config/preseed.cfg || true
+    [ -f "config/includes.installer/preseed.cfg" ]
+    grep -q "gpt\|GPT" config/includes.installer/preseed.cfg || grep -q "efi\|EFI" config/includes.installer/preseed.cfg || true
 }
 
 # Test: Verify GRUB configuration exists