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/unit/build-iso_comprehensive_test.bats

@@ -137,7 +137,7 @@
 }
 
 @test "config/preseed.cfg exists" {
-    [ -f "/workspace/config/preseed.cfg" ]
+    [ -f "/workspace/config/includes.installer/preseed.cfg" ]
 }
 
 @test "config/hooks directory exists" {