fix(ci): remove pnpm cache — no lockfile on main yet

setup-node's cache: pnpm requires pnpm-lock.yaml to exist. The lockfile
is coming in PR #15 but isn't on main yet. Remove caching for now —
it'll be re-enabled once the lockfile lands.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -18,7 +18,6 @@ jobs:
       - uses: actions/setup-node@v4
         with:
           node-version: 20
-          cache: pnpm
 
       - name: Install dependencies
         run: pnpm install --frozen-lockfile
@@ -40,7 +39,6 @@ jobs:
       - uses: actions/setup-node@v4
         with:
           node-version: 20
-          cache: pnpm
 
       - name: Install dependencies
         run: pnpm install --frozen-lockfile
@@ -60,7 +58,6 @@ jobs:
       - uses: actions/setup-node@v4
         with:
           node-version: 20
-          cache: pnpm
 
       - name: Install dependencies
         run: pnpm install --frozen-lockfile