ci: Skip build on doc-only changes and cancel superseded runs

Every push to main triggered a full macOS build, including
docs-only commits like README tweaks or CLAUDE.md updates. Add
\`paths-ignore\` for markdown, LICENSE, .gitignore, and the
uninstall script so CI fires only on changes that can actually break
the build, and add a concurrency group so a new push cancels any
in-progress run on the same ref instead of letting stale builds eat
runner minutes.

Author: cuiko

.github/workflows/build.yml

@@ -3,7 +3,23 @@ name: build
 on:
   push:
     branches: [main]
+    paths-ignore:
+      - '**.md'
+      - 'LICENSE'
+      - '.gitignore'
+      - 'uninstall.sh'
   pull_request:
+    paths-ignore:
+      - '**.md'
+      - 'LICENSE'
+      - '.gitignore'
+      - 'uninstall.sh'
+
+# Cancel in-progress runs when a new commit supersedes them — no point
+# building stale code while a newer push is already queued.
+concurrency:
+  group: build-${{ github.ref }}
+  cancel-in-progress: true
 
 jobs:
   build: