fix: invert reuseExistingServer and remove CI="" workaround

The playwright.config.ts had `reuseExistingServer: !!process.env.CI`
which meant CI would reuse (expect) an existing server while local
dev would start one. This is backwards — in CI Playwright should
manage the server, and in local dev you likely already have one
running.

Flip to `!process.env.CI` and remove the `CI: ""` env override
from the workflow.

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

.github/workflows/pr-e2e.yml

@@ -56,7 +56,6 @@ jobs:
       - name: Run e2e tests
         env:
           PAPERCLIP_E2E_SKIP_LLM: "true"
-          CI: ""
         run: pnpm run test:e2e
 
       - name: Upload Playwright report

tests/e2e/playwright.config.ts

@@ -25,7 +25,7 @@ export default defineConfig({
   webServer: {
     command: `pnpm paperclipai run`,
     url: `${BASE_URL}/api/health`,
-    reuseExistingServer: !!process.env.CI,
+    reuseExistingServer: !process.env.CI,
     timeout: 120_000,
     stdout: "pipe",
     stderr: "pipe",