fix(ci): run CI on Node 22.x and correct undici 8 Node floor (#1563)

undici 8.5.0 requires Node >= 22.19.0 (its declared engines), so CI's
Node 20.x could not load undici's CacheStorage (missing
webidl.util.markAsUncloneable) and the proxy integration tests failed.
Bump the CI setup-node to 22.x to match the repo's Node floor.

Reconcile the declared floor with undici's real requirement: bump root
engines.node to >=22.19.0 and correct the CLI README note (undici 8.5.0
needs >=22.19.0, not >=22.7.5). Also document proxy support in the web
README and clarify that a caller-supplied eventSourceInit.fetch bypasses
the proxy wrapper by design.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S3fTN8H3R8YV4yUGvZjYnX

.github/workflows/main.yml

@@ -13,7 +13,7 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@v6
         with:
-          node-version: '20.x'
+          node-version: '22.x'
           cache: 'npm'
 
       - name: Install dependencies (root + all clients)