ci: target Gitea Ubuntu and native Apple Silicon runners

Move workflows into Gitea's workflow directory and use the supplied ubuntu-latest and macos-arm64 labels. Provision the Android SDK, require KVM without changing host permissions, support SHA-256 checkout and Gitea artifact uploads, and isolate scratch output on persistent runners. Skip untrusted fork PRs and keep release builds unsigned because Gitea ignores GitHub environment approval gates. Local workflow lint, repository invariants, ten preflight regressions, eight evidence-verifier tests, inline shell checks, scratch cleanup and SHA-256 fetch checks pass; remote runner execution remains unverified.

README.md

@@ -18,6 +18,7 @@ reconnect or erase local browsing data and switch installations.
 - [Android build](docs/build-android.md) / [iOS build](docs/build-ios.md)
 - [Compatibility and authentication](docs/server-compatibility.md)
 - [Release process](docs/release-process.md)
+- [Gitea CI runners](docs/ci-runners.md)
 - [Maintainer continuation](docs/continuation.md)
 
 MIT licensed. Upstream notices are preserved in `THIRD_PARTY_NOTICES.md`.