chore: bump GitHub Actions versions and add skills improvement plan

Update all actions/checkout, setup-go, setup-node, cache, upload-artifact,
and download-artifact to latest major versions. Add plan.md with roadmap
for skill fixes and two new skills (mt:commit-push, mt:backfill-tags).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/hugo.yml

@@ -26,17 +26,17 @@ jobs:
       TZ: Asia/Ho_Chi_Minh
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           submodules: recursive
           fetch-depth: 0
       - name: Setup Go
-        uses: actions/setup-go@v5
+        uses: actions/setup-go@v6
         with:
           go-version: ${{ env.GO_VERSION }}
           cache: false
       - name: Setup Node.js
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
           node-version: ${{ env.NODE_VERSION }}
       # - name: Setup Pages
@@ -72,7 +72,7 @@ jobs:
           git config core.quotepath false
       - name: Cache restore
         id: cache-restore
-        uses: actions/cache/restore@v4
+        uses: actions/cache/restore@v5
         with:
           path: ${{ runner.temp }}/hugo_cache
           key: hugo-${{ github.run_id }}
@@ -87,12 +87,12 @@ jobs:
             --cacheDir "${{ runner.temp }}/hugo_cache"
       - name: Cache save
         id: cache-save
-        uses: actions/cache/save@v4
+        uses: actions/cache/save@v5
         with:
           path: ${{ runner.temp }}/hugo_cache
           key: ${{ steps.cache-restore.outputs.cache-primary-key }}
       - name: Upload artifact
-        uses: actions/upload-pages-artifact@v3
+        uses: actions/upload-pages-artifact@v4
         with:
           path: ./public
   # deploy:
@@ -111,7 +111,7 @@ jobs:
     needs: build
     steps:
       - name: Download artifact
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v8
         with:
           name: github-pages
           path: .