fixup dapp CI config (#9947)

_incidental_

## Description
documentation CI is [failing](https://github.com/Agoric/agoric-sdk/actions/runs/10513357847/job/29128771880?pr=9945) since Agoric/documentation#912

This fixes it

### Security Considerations


### Scaling Considerations


### Documentation Considerations


### Testing Considerations


### Upgrade Considerations

.github/actions/restore-node/action.yml

@@ -111,6 +111,10 @@ runs:
       shell: bash
       run: git config --global url."https://github.com/".insteadOf ssh://[email protected]/
 
+    # Before setup-node because that action runs `yarn cache dir`. See https://github.com/actions/setup-node/issues/480#issuecomment-1915448139
+    - run: corepack enable
+      shell: bash
+
     - uses: actions/setup-node@v4
       with:
         node-version: ${{ inputs.node-version }}
@@ -122,9 +126,6 @@ runs:
     - uses: kenchan0130/actions-system-info@master
       id: system-info
 
-    - run: corepack enable
-      shell: bash
-
     - name: restore built files
       id: built
       uses: actions/cache@v4

.github/workflows/README.md

@@ -0,0 +1,8 @@
+# Github Workflows
+
+We use these for CI. Some other workflows that we aren't currently using are:
+
+* [dapp template](https://github.com/Agoric/agoric-sdk/blob/7a08c6af641c62d8b0ef4a0c35090b6216a5be34/.github/workflow-templates/test-dapp.yml#L1)
+* [ag-solo on xs](https://github.com/Agoric/agoric-sdk/blob/7a08c6af641c62d8b0ef4a0c35090b6216a5be34/.github/workflows/ag-solo-xs.yml.DISABLED#L1)
+* [Test Dapp Treasury](https://github.com/Agoric/agoric-sdk/blob/7a08c6af641c62d8b0ef4a0c35090b6216a5be34/.github/workflows/test-dapp-treasury.yml.DISABLED#L1)
+* [Update Hackathon branch](https://github.com/Agoric/agoric-sdk/blob/7a08c6af641c62d8b0ef4a0c35090b6216a5be34/.github/workflows/update-hackathon-branch.yml.DISABLED#L1)

.github/workflows/test-documentation.yml

@@ -26,12 +26,6 @@ jobs:
           node-version: ${{ matrix.node-version }}
           path: ./agoric-sdk
 
-      - name: yarn link
-        run: |
-          yarn link-cli ~/bin/agoric
-          echo "/home/runner/bin" >> $GITHUB_PATH
-        working-directory: ./agoric-sdk
-
       # Select a branch on dapp to test against by adding text to the body of the
       # pull request. For example: #dapp-encouragement-branch: zoe-release-0.7.0
       # The default is 'main'
@@ -60,8 +54,10 @@ jobs:
           path: dapp
           ref: ${{steps.get-branch.outputs.result}}
 
-      - name: Agoric install in dapp
-        run: agoric install
+      - name: point dapp to agoric-SDK HEAD
+        # This assumes the dapp uses Yarn 4 and its other deps are compatible
+        # with the versions in the agoric-sdk packages.
+        run: yarn link ../agoric-sdk --all --relative
         working-directory: ./dapp
 
       - name: yarn build in dapp