2 new efficiency skills to reduce cost and wasted cpu cycles in github actions and codespaces (#1766)

* fresh pull of updated stage with 2 skills and updated README only

* adjusted skills after feedback and used skill analyser to review

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Co-authored-by: Mark Heynes <mutl3y@heynes.biz>

skills/github-actions-efficiency/references/patterns.md

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+# Canonical Patterns
+
+Load this reference only when you need concrete examples during implementation.
+
+## Dependency Cache
+
+```yaml
+- uses: actions/cache@v4
+  with:
+    path: ~/.npm
+    key: ${{ runner.os }}-node-${{ hashFiles('**/package-lock.json') }}
+    restore-keys: |
+      ${{ runner.os }}-node-
+```
+
+Adapt the cache path and invalidation file to the repo's ecosystem.
+
+## Cancel Stale Runs
+
+```yaml
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+```
+
+## Scope Triggers
+
+```yaml
+on:
+  push:
+    paths:
+      - "src/**"
+      - "tests/**"
+      - "package.json"
+```
+
+Use `paths-ignore` when exclusion is easier to maintain than inclusion.
+
+## Job-Level Changed-File Gating
+
+Use a small change-detection step that emits explicit outputs such as:
+
+- `docs_relevant`
+- `runtime_relevant`
+- `compat_relevant`
+- `run_tests`
+
+Gate downstream jobs on those outputs when event-level filters are not expressive enough.
+
+## Matrix Reduction
+
+Use the minimum matrix that matches the decision:
+
+- Full matrix on release
+- Reduced compatibility matrix on sensitive runtime surfaces
+- Single representative leg for ordinary code changes
+
+## Optional Write-Back Job
+
+Use label-driven or manual triggers for jobs that mutate the PR branch, such as formatting bots.