ci: release MCP binaries + path-gate heavyweight jobs

- build-cli.sh also builds containr-mcp-<os>-<arch> per platform; the
  existing containr-* asset glob in the release job picks them up
- paths-filter 'changes' job gates backend/frontend/smoke/docker on PRs —
  docs-only PRs skip ~10min of compute; skipped checks satisfy protection
- docker job requires success() explicitly (skipped parents would pass
  '!= failure'); release requires docker success to never release stale images

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

.github/workflows/ci.yml

@@ -17,6 +17,27 @@ permissions:
   contents: read
 
 jobs:
+  # Cheap gate so docs-only/roadmap PRs skip the heavyweight jobs. Skipped
+  # required checks still satisfy branch protection ("skipped" = success).
+  changes:
+    name: Changed paths
+    runs-on: ubuntu-latest
+    outputs:
+      code: ${{ steps.filter.outputs.code }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dorny/paths-filter@v3
+        id: filter
+        with:
+          filters: |
+            code:
+              - 'app/**'
+              - 'infra/**'
+              - 'docker-compose.yml'
+              - 'install.sh'
+              - '.github/workflows/**'
+              - 'docs/api/**'
+
   infra:
     name: Infra (Compose + Scripts)
     runs-on: ubuntu-latest
@@ -48,6 +69,8 @@ jobs:
   backend:
     name: Backend (Go)
     runs-on: ubuntu-latest
+    needs: changes
+    if: needs.changes.outputs.code == 'true' || github.event_name != 'pull_request'
     defaults:
       run:
         working-directory: app/backend
@@ -70,6 +93,8 @@ jobs:
   frontend:
     name: Frontend (React)
     runs-on: ubuntu-latest
+    needs: changes
+    if: needs.changes.outputs.code == 'true' || github.event_name != 'pull_request'
     defaults:
       run:
         working-directory: app/frontend
@@ -100,6 +125,8 @@ jobs:
     name: Compose boot smoke
     runs-on: ubuntu-latest
     timeout-minutes: 20
+    needs: changes
+    if: needs.changes.outputs.code == 'true' || github.event_name != 'pull_request'
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -137,6 +164,7 @@ jobs:
     name: Build & publish images
     runs-on: ubuntu-latest
     needs: [backend, frontend, smoke]
+    if: needs.backend.result == 'success' && needs.frontend.result == 'success' && needs.smoke.result == 'success'
     permissions:
       contents: read
       packages: write
@@ -227,7 +255,7 @@ jobs:
     name: Release
     runs-on: ubuntu-latest
     needs: [docker]
-    if: github.event_name != 'pull_request'
+    if: github.event_name != 'pull_request' && needs.docker.result == 'success'
     permissions:
       contents: write
       packages: write