fix: use git rev-parse for short SHA in build-dev

$GITEA_SHA is unset on Gitea runners — the nav showed "dev-" with an
empty SHA. git rev-parse --short HEAD works regardless of runner env vars.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/helpers.test.js

@@ -156,4 +156,11 @@ describe('CI workflow regressions', () => {
     // BUILD_VERSION was never passed to docker build.
     expect(ciYml).toContain('BUILD_VERSION')
   })
+
+  it('short SHA is computed with git rev-parse, not $GITEA_SHA (which is empty)', () => {
+    // Regression: ${GITEA_SHA::7} expands to "" on Gitea runners — nav showed "dev-".
+    // git rev-parse --short HEAD works regardless of which env vars the runner sets.
+    expect(ciYml).toContain('git rev-parse --short HEAD')
+    expect(ciYml).not.toContain('GITEA_SHA')
+  })
 })