ci: move CI back to GitHub Actions, add a macOS job

The Gitea migration left CI on the self-hosted instance with every
cache commented out and the CD workflow broken at parse time
(kiname:). CI runs on GitHub-hosted runners again:

- Linux: ubuntu-latest with the rust toolchain from
  dtolnay/rust-toolchain (the self-hosted runner's custom
  RUSTUP_HOME/CARGO_HOME lines are gone) and the cargo + FFmpeg caches
  re-enabled.
- Windows: windows-latest with msys2/setup-msys2 provisioning the
  UCRT64 environment (the Gitea runner had it preinstalled); the
  msys2 {0} shell, the GNU-target MSYS2 Rust, the
  OCIO_RS_NO_MSVC_INCLUDES gate and the -lmsvcrt link-order workaround
  carry over, as do both caches.
- macOS: new job on macos-14 (Apple Silicon) — Homebrew deps, vendored
  static OCIO, cached FFmpeg, cargo check + the full suite with the
  same retry-once flake policy.

CD fixes: the kiname: typo that kept the workflow from parsing, the
Warp runner labels become the standard GitHub ones, and all eight
commented-out cache blocks are restored. The gpui submodule URL
follows the move to GitHub, the README badge points at the GitHub
workflow, and the .gitea directory is dropped.

.github/actionlint.yaml

@@ -0,0 +1,8 @@
+# actionlint configuration: whitelist the WarpBuild runner labels used by
+# the CI/CD workflows so `actionlint ci.yml` / `actionlint cd.yml` passes.
+# See https://github.com/rhysd/actionlint/blob/main/docs/config.md
+self-hosted-runner:
+  labels:
+    - warp-ubuntu-latest-x64-8x
+    - warp-windows-latest-x64-16x
+    - warp-macos-26-arm64-6x