ci: add Gitea Actions workflow for check, fmt, clippy

Three parallel jobs on the rust runner:
- cargo check --workspace
- rustfmt --check on all .rs files
- cargo clippy --workspace -- -D warnings

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yaml

@@ -0,0 +1,25 @@
+name: CI
+
+on:
+  push:
+    branches: ["*"]
+  pull_request:
+
+jobs:
+  check:
+    runs-on: rust
+    steps:
+      - uses: actions/checkout@v4
+      - run: cargo check --workspace
+
+  fmt:
+    runs-on: rust
+    steps:
+      - uses: actions/checkout@v4
+      - run: rustfmt --check --edition 2024 $(find crates -name '*.rs')
+
+  clippy:
+    runs-on: rust
+    steps:
+      - uses: actions/checkout@v4
+      - run: cargo clippy --workspace -- -D warnings