ci: cache Go toolchains across main runs

Reduce repeated Go toolchain downloads while preserving main/manual-only workflow execution.

.github/workflows/repository-hygiene.yml

@@ -89,6 +89,11 @@ jobs:
       # The Go build embeds internal/server/webassets/dist, so Vite runs first.
       - name: Build the Browser client
         run: npm run build:client
+      - name: Cache Go toolchain
+        uses: actions/cache@v4
+        with:
+          path: ${{ runner.tool_cache }}/go
+          key: ${{ runner.os }}-go-1.26.6
       - name: Set up Go
         uses: actions/setup-go@v6
         with:
@@ -114,6 +119,11 @@ jobs:
         with:
           node-version-file: .node-version
           cache: npm
+      - name: Cache Go toolchain
+        uses: actions/cache@v4
+        with:
+          path: ${{ runner.tool_cache }}/go
+          key: ${{ runner.os }}-go-1.26.6
       - name: Set up Go
         uses: actions/setup-go@v6
         with:
@@ -158,6 +168,11 @@ jobs:
         with:
           node-version-file: .node-version
           cache: npm
+      - name: Cache Go toolchain
+        uses: actions/cache@v4
+        with:
+          path: ${{ runner.tool_cache }}/go
+          key: ${{ runner.os }}-go-1.26.6
       - name: Set up Go
         uses: actions/setup-go@v6
         with: