fix(ci): remove invalid --jobs flag from pnpm install

pnpm 11 has no --jobs option for install; the stray positional '1'
flipped the command into 'add' mode, which then rejected both
--frozen-lockfile and --jobs ('Unknown options', 'pnpm help add').
Reproduced locally, fixed, verified install succeeds. Add contract
regression test.

.gitea/workflows/ci.yaml

@@ -29,7 +29,7 @@ jobs:
         run: node scripts/check-node-toolchain.mjs
 
       - name: Install dependencies
-        run: pnpm install --frozen-lockfile --jobs 1
+        run: pnpm install --frozen-lockfile
 
       - name: Lint
         run: pnpm biome:lint