Simplify workflows: drop setup-node/pnpm-action, use corepack

The runner containers use node:20-bookworm which already has Node
and corepack. Remove actions/setup-node and pnpm/action-setup
which hang in Gitea Actions. Use corepack enable + pnpm directly.
Also fix preview comment to use Gitea API instead of github-script.

.gitea/workflows/ci.yaml

@@ -41,12 +41,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - uses: pnpm/action-setup@v4
-
-      - uses: actions/setup-node@v4
-        with:
-          node-version: 20
-          cache: pnpm
+      - run: corepack enable && corepack prepare pnpm@latest --activate
 
       - run: pnpm install --frozen-lockfile
 
@@ -62,12 +57,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - uses: pnpm/action-setup@v4
-
-      - uses: actions/setup-node@v4
-        with:
-          node-version: 20
-          cache: pnpm
+      - run: corepack enable && corepack prepare pnpm@latest --activate
 
       - run: pnpm install --frozen-lockfile