ci: cancel superseded PR runs; skip build on docs-only PRs (#440)

Two workflow tweaks aimed at the partial-miss tier (~10-13min) build
that dominates everyday PR iteration:

1. Concurrency cancellation - in-progress runs on the same PR branch
   are cancelled when a new push arrives. main pushes are not cancelled
   (cancel-in-progress is gated on github.event_name == 'pull_request')
   so each merged commit still gets its own verification pass.

2. paths-ignore on the Build and Test trigger - PRs that touch only
   markdown files or Docs/ skip the full build/test cycle entirely.
   PR Validation (villint) keeps its current trigger so doc PRs still
   pass through linting.

If 'Build and Test VillageSQL' is configured as a required status check
on main, doc-only PRs may report it as missing rather than passing -
will need a no-op fallback job in that case.

Author: pyshx

.github/workflows/build.yml

@@ -3,8 +3,20 @@ name: Build and Test
 on:
   push:
     branches: [ main ]
+    paths-ignore:
+      - '**.md'
+      - 'Docs/**'
   pull_request:
     branches: [ main ]
+    paths-ignore:
+      - '**.md'
+      - 'Docs/**'
+
+# Cancel superseded runs on the same PR branch. main pushes are not cancelled
+# so each merged commit gets its own pass.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
 jobs:
   build-and-test:

.github/workflows/validate-pr.yml

@@ -9,6 +9,10 @@ on:
     - reopened
     - synchronize
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   validate:
     name: Validate PR