ci: upgrade yarn to 4.x.x (#1390)

This PR will update yarn 3 to yarn 4, with the following addition:
1. delegate subpackages to reference the root `package.json` for yarn
version (before, we had specified them individually)
2. combined GHA action for setting up yarn (before almost all of our
workflows will have redundant setup steps)
3. add a package minimium age gate set to 3 days (to protect against
supply-chain attacks) <-- this is the main reason this PR exists

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Touches all CI install paths and adds an npm minimum-age gate that can
fail builds on brand-new dependencies; no runtime SDK logic changes in
the diff shown.
> 
> **Overview**
> Upgrades the monorepo from **Yarn 3 to Yarn 4** (`yarn@4.15.0` via
root `packageManager` and `.yarn/releases`), and centralizes how CI
installs Node/Yarn.
> 
> **CI consolidation:** Dozens of workflows swap `actions/setup-node`
plus ad‑hoc `corepack enable` for the composite
**`./actions/setup-yarn`** (Node + Corepack). Release setup
(`actions/setup-release-node`) uses the same action.
**`dependency-scan.yml`** drops its standalone Corepack step.
> 
> **Supply chain:** **`.yarnrc.yml`** adds **`npmMinimalAgeGate: 4320`**
(~3 days) so newly published npm packages cannot be installed
immediately.
> 
> **Targeted CI fixes:** **Electron** jobs pin **Node 24.15** (matrix
and examples) because newer runner Node breaks Electron installs.
**`server-ai.yml`** tightens example builds by removing the `-A` flag
from `yarn workspaces foreach`.
> 
> **Yarn 4 cleanup:** The vendored
**`.yarn/plugins/@yarnpkg/plugin-workspace-tools.cjs`** is removed
(behavior expected from Yarn 4 / repo config). Workspace packages stop
declaring their own `packageManager`; they inherit the root pin.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
9350b92. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: joker23

.github/workflows/akamai-base.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+      - uses: ./actions/setup-yarn
       - id: shared
         name: Shared CI Steps
         uses: ./actions/ci

.github/workflows/akamai-edgekv.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+      - uses: ./actions/setup-yarn
       - id: shared
         name: Shared CI Steps
         uses: ./actions/ci

.github/workflows/akamai-edgeworker-sdk.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+      - uses: ./actions/setup-yarn
         with:
           node-version: 24.x
           registry-url: 'https://registry.npmjs.org'

.github/workflows/browser-nightly.yaml

@@ -11,7 +11,7 @@ jobs:
       id-token: write
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+      - uses: ./actions/setup-yarn
         with:
           node-version: 24
       - uses: ./actions/run-example

.github/workflows/browser.yml

@@ -23,7 +23,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+      - uses: ./actions/setup-yarn
         with:
           node-version: ${{ matrix.version }}
           registry-url: 'https://registry.npmjs.org'
@@ -135,7 +135,7 @@ jobs:
       id-token: write
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+      - uses: ./actions/setup-yarn
         with:
           node-version: 24
       - uses: ./actions/run-example

.github/workflows/cloudflare.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+      - uses: ./actions/setup-yarn
       - id: shared
         name: Shared CI Steps
         uses: ./actions/ci

.github/workflows/combined-browser.yml

@@ -23,7 +23,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+      - uses: ./actions/setup-yarn
         with:
           node-version: ${{ matrix.version }}
           registry-url: 'https://registry.npmjs.org'

.github/workflows/common.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+      - uses: ./actions/setup-yarn
         with:
           node-version: 24.x
           registry-url: 'https://registry.npmjs.org'

.github/workflows/dependency-scan.yml

@@ -12,13 +12,10 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
-      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+      - uses: ./actions/setup-yarn
         with:
           node-version: 24.x
 
-      - name: Enable corepack
-        run: corepack enable
-
       - name: Install released package dependencies
         run: yarn workspaces focus $(node scripts/released-packages.js)
         env:

.github/workflows/electron.yaml

@@ -26,7 +26,7 @@ jobs:
         version: [24.15]
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+      - uses: ./actions/setup-yarn
         with:
           node-version: ${{ matrix.version }}
           registry-url: 'https://registry.npmjs.org'
@@ -81,7 +81,7 @@ jobs:
       id-token: write
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
+      - uses: ./actions/setup-yarn
         with:
           node-version: 24.15
       - uses: ./actions/run-example