fix(ci): use corepack instead of pnpm/action-setup

pnpm/action-setup@v4 ran at the repo root (uses: steps ignore
defaults.run.working-directory) where there is no package.json, so it couldn't
read the pnpm version → "No pnpm version specified". Use corepack (bundled with
node) in the install step, which reads each app's own packageManager — matching
the Dockerfiles. Verified in the runner's container: corepack enable + frozen
install succeeds for every app.

.gitea/workflows/ci.yml

@@ -25,19 +25,17 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      # `uses:` steps ignore defaults.run.working-directory, so action-setup runs
-      # at the repo root — which has no package.json (per-app monorepo). Pin the
-      # version explicitly instead of reading packageManager.
-      - uses: pnpm/action-setup@v4
-        with:
-          version: 9
-
       - uses: actions/setup-node@v4
         with:
           node-version: 22
 
+      # Use corepack (bundled with node) instead of pnpm/action-setup: it reads
+      # each app's packageManager from its own package.json, which is what this
+      # per-app monorepo (no root package.json) needs.
       - name: Install
-        run: pnpm install --frozen-lockfile
+        run: |
+          corepack enable
+          pnpm install --frozen-lockfile
 
       - name: Typecheck
         run: pnpm typecheck
@@ -49,13 +47,12 @@ jobs:
         working-directory: services/platform-api
     steps:
       - uses: actions/checkout@v4
-      - uses: pnpm/action-setup@v4
-        with:
-          version: 9
       - uses: actions/setup-node@v4
         with:
           node-version: 22
       - name: Install
-        run: pnpm install --frozen-lockfile
+        run: |
+          corepack enable
+          pnpm install --frozen-lockfile
       - name: Test
         run: pnpm test