Replace Checkout action with Cache action for build file caching

Convert the composite action from Checkout to Cache that:
- Stores build files under ~/.cache/ by default
- Auto-generates cache keys from lockfile hashes
- Uses actions/cache@v5 for restore and save
- Supports custom path, key, key-file, and restore-keys inputs
- Exposes cache-hit output for conditional workflow steps

action.yaml

@@ -0,0 +1,48 @@
+name: 'Cache'
+description: 'Cache build files under ~/.cache/ using a key file hash'
+inputs:
+  path:
+    description: 'A list of files, directories, or wildcard patterns to cache. Defaults to ~/.cache/'
+    required: false
+    default: '~/.cache/'
+  key:
+    description: 'An explicit key for a cache entry. Defaults to runner.os-cache-{hash}'
+    required: false
+  key-file:
+    description: 'The file to hash for generating the cache key. Used if key is not provided.'
+    required: false
+    default: ''
+  restore-keys:
+    description: 'An ordered list of prefix-matched keys for restoring stale cache'
+    required: false
+    default: ''
+outputs:
+  cache-hit:
+    description: 'A boolean value to indicate an exact match was found for the key'
+    value: ${{ steps.restore-cache.outputs.cache-hit }}
+runs:
+  using: "composite"
+  steps:
+    - name: Generate cache key
+      id: gen-key
+      if: inputs.key == ''
+      shell: bash
+      run: |
+        if [ -n "${{ inputs.key-file }}" ]; then
+          KEY_HASH=$(sha256sum "${{ inputs.key-file }}" | awk '{print $1}')
+        else
+          KEY_HASH=$(find . -type f \( -name "*.csproj" -o -name "*.sln" -o -name "package.json" -o -name "package-lock.json" -o -name "yarn.lock" -o -name "pnpm-lock.yaml" -o -name "Cargo.lock" -o -name "requirements.txt" -o -name "Pipfile.lock" -o -name "go.mod" -o -name "go.sum" -o -name "pom.xml" -o -name "build.gradle*" -o -name "Gemfile.lock" -o -name "composer.lock" \) -exec sha256sum {} + 2>/dev/null | sha256sum | awk '{print $1}')
+          if [ "$KEY_HASH" = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855" ]; then
+            KEY_HASH=$(find . -type f -not -path './.git/*' -not -path './.cache/*' | sort | xargs sha256sum 2>/dev/null | sha256sum | awk '{print $1}')
+          fi
+        fi
+        echo "cache-key=${{ runner.os }}-cache-${KEY_HASH}" >> $GITHUB_OUTPUT
+
+    - name: Restore cache
+      id: restore-cache
+      uses: actions/cache@v5
+      with:
+        path: ${{ inputs.path }}
+        key: ${{ inputs.key != '' && inputs.key || steps.gen-key.outputs.cache-key }}
+        restore-keys: ${{ inputs.restore-keys != '' && inputs.restore-keys || format('{0}-cache-', runner.os) }}
+