ci: faster pipelines with checks-only ci and registry build cache

Drop duplicate Next.js builds from CI, skip release commits, and cancel stale runs.

Cache pnpm on runners and use buildx registry cache for releases.

.github/workflows/ci.yml

@@ -3,10 +3,21 @@ name: CI
 on:
   push:
     branches: [main]
+    paths-ignore:
+      - "**/*.md"
+      - "docs/**"
   pull_request:
+    paths-ignore:
+      - "**/*.md"
+      - "docs/**"
+
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 
 jobs:
   checks:
+    if: "github.event_name == 'pull_request' || !startsWith(github.event.head_commit.message, 'chore: release')"
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
@@ -27,33 +38,3 @@ jobs:
       - run: pnpm lint
 
       - run: pnpm format:check
-
-  build:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-
-      - uses: pnpm/action-setup@v4
-        with:
-          version: 10.33.3
-
-      - uses: actions/setup-node@v4
-        with:
-          node-version: 24
-          cache: pnpm
-
-      - run: pnpm install --frozen-lockfile
-
-      - uses: actions/cache@v4
-        with:
-          path: .next/cache
-          key: ${{ runner.os }}-nextjs-${{ hashFiles('pnpm-lock.yaml') }}-${{ hashFiles('src/**/*.ts', 'src/**/*.tsx', 'src/**/*.css') }}
-          restore-keys: |
-            ${{ runner.os }}-nextjs-${{ hashFiles('pnpm-lock.yaml') }}-
-            ${{ runner.os }}-nextjs-
-
-      - run: pnpm build
-        env:
-          DATABASE_URL: postgres://placeholder:placeholder@localhost:5432/placeholder
-          AUTH_SECRET: ci-placeholder-secret
-          NEXT_PUBLIC_APP_URL: http://localhost:3000