feat(ci): add Forgejo Actions workflow for CI checks

Creates .forgejo/workflows/ci.yml that runs the repo's single-source-of-truth
check gate (./script/checks) on every push to main and every pull request.

Triggers: push to main, pull_request
Concurrency: group by workflow+ref, cancel-in-progress true
Steps: actions/checkout@v4, actions/setup-node@v4 (node-version-file: .nvmrc),
corepack + pnpm i --frozen-lockfile, then ./script/checks.

Part of bead mailship-e77.

.forgejo/workflows/ci.yml

@@ -0,0 +1,31 @@
+# Forgejo Actions CI — runs the repo's check gate on every push/PR
+# Single source of truth: ./script/checks defines what "passing CI" means.
+---
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  checks:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version-file: .nvmrc
+
+      - name: Install dependencies
+        run: |
+          corepack enable
+          pnpm i --frozen-lockfile
+
+      - name: Run check gate
+        run: ./script/checks