Add Gitea Actions CI workflow (#53)

Follow-up to [PR #51](#51) — addresses sneak's review comment requesting Gitea Actions to run the checks.

## Changes

Adds `.gitea/workflows/check.yml` that runs `docker build .` on every push, matching the standard CI pattern used across all `sneak/*` repos (identical to [chat](https://git.eeqj.de/sneak/chat/src/branch/main/.gitea/workflows/check.yml) and [dnswatcher](https://git.eeqj.de/sneak/dnswatcher/src/branch/main/.gitea/workflows/check.yml)).

Since the Dockerfile already runs `make check` (fmt-check, lint, test), a successful build implies all checks pass.

The `actions/checkout` action is pinned by SHA (`@11bd71901bbe5b1630ceea73d27597364c9af683` = v4.2.2) per REPO_POLICIES.

## Verification

`docker build .` passes clean.

Reviewed-on: #53
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>
Co-committed-by: clawbot <sneak+clawbot@sneak.cloud>

.gitea/workflows/check.yml

@@ -0,0 +1,9 @@
+name: check
+on: [push]
+jobs:
+    check:
+        runs-on: ubuntu-latest
+        steps:
+            # actions/checkout v4.2.2, 2026-03-16
+            - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
+            - run: docker build .