Replace Drone CI with Gitea Actions workflow

Drone server is gone; CI now lives in .gitea/workflows/build.yml.
Three jobs: GDScript validation (godot --headless --import + error
grep), desktop matrix (Windows / Linux / macOS), and Android (JDK 17
+ SDK installed at runtime, keystore from ANDROID_KEYSTORE_BASE64
secret or generated). Build only — Butler / itch.io deploy not wired.

Notes in .gitea/workflows/README.md cover the Godot-4 pre-reqs
(macOS preset to add, Linux/X11Debug likely renamed on first 4.x
save, Docker image tag) and how to plug Butler back in later.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -149,10 +149,12 @@ re-author it with Godot 4's built-in `theme_override_constants/outline_size`
 
 ## CI
 
-Build pipeline is in `releases/.drone.yml`. The Docker images still pin
-`barichello/godot-ci:3.3.2` and a custom `devcrea/godot-ci:3.3.2-android`
-— **bump these to a 4.x image** before relying on CI builds again. Butler
-push targets `dev-crea/ahog:windows|android|linux|mac` on itch.io.
+Gitea Actions workflow at `.gitea/workflows/build.yml`, documented in
+`.gitea/workflows/README.md`. Three jobs: GDScript validation
+(`godot --headless --import` + error grep), desktop matrix
+(Windows / Linux / macOS), and Android. Build only — no Butler / itch.io
+deploy currently wired (channels used historically were
+`dev-crea/ahog:windows|android|linux|mac`). Drone pipeline removed.
 
 Branches: default `dev`, releases from `main`. Long-running migration work
 on `feature/godot-migration`.