Add Gitea Actions CI workflow

Runs on push to main and pull requests against main:
- go mod download + verify
- make lint (go vet)
- make build
- make test (race detector)

Uses actions/setup-go@v5 with built-in module caching, Go 1.22.
Workflow times out at 5 minutes per the acceptance criteria.

Closes #5.

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

.gitea/workflows/ci.yml

@@ -0,0 +1,33 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  ci:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: "1.22"
+          cache: true
+
+      - name: Verify modules
+        run: go mod download && go mod verify
+
+      - name: Vet
+        run: make lint
+
+      - name: Build
+        run: make build
+
+      - name: Test
+        run: make test