feat: migrate CI to Codeberg Actions and add self-hosted runner config

- Added .forgejo/workflows/ci.yml for thin CI orchestration.
- Configured Dockerized Codeberg runner with Nix support in codeberg-runner/.
- Added systemd service for persistent runner execution.
- Added GEMINI.md for project CI/Nix conventions.
- Added Taskfile task for Linux release builds.

.gitignore

@@ -60,3 +60,5 @@ linux/flutter/generated_plugins.cmake
 .task
 
 *.log
+runner-data/
+codeberg-runner/runner-data/