feat: rename package to lthn/php for Packagist distribution

- composer.json name: core/php → lthn/php
- replace directive keeps backward compat with core/php consumers
- .gitattributes excludes Go code, Docker, tests, build files from dist

Co-Authored-By: Virgil <virgil@lethean.io>

.gitattributes

@@ -0,0 +1,16 @@
+# Exclude non-PHP files from Composer dist archives
+*.go              export-ignore
+go.mod            export-ignore
+go.sum            export-ignore
+cmd/              export-ignore
+bin/              export-ignore
+docker/           export-ignore
+changelog/        export-ignore
+tests/            export-ignore
+.core/            export-ignore
+.forgejo/         export-ignore
+phpunit.xml       export-ignore
+phpstan.neon      export-ignore
+qa.yaml           export-ignore
+CLAUDE.md         export-ignore
+CONTRIBUTING.md   export-ignore

composer.json

@@ -1,5 +1,5 @@
 {
-    "name": "core/php",
+    "name": "lthn/php",
     "description": "Modular monolith framework for Laravel - event-driven architecture with lazy module loading",
     "keywords": [
         "laravel",
@@ -88,5 +88,8 @@
         }
     },
     "minimum-stability": "stable",
-    "prefer-stable": true
+    "prefer-stable": true,
+    "replace": {
+        "core/php": "self.version"
+    }
 }