fix(ci): build artifacts in integration-tests job

PR #4093 split build and integration-tests into parallel CI jobs but the new
integration-tests job only ran `npm ci`, leaving tests without the compiled
artifacts they spawn at runtime. That caused 8 failures on main (run 24325713845):

- e2e-headless, e2e-smoke, pack-install — throw "dist/loader.js not found"
- 4 web-session-parity / web-live-state tests — "session manager module not
  found; checked=packages/pi-coding-agent/dist/core/session-manager.js"
- web-mode-onboarding — "sh: 1: next: not found" when the test shells
  `npm run build:web-host` at runtime (web/node_modules/.bin/next absent)

Add `npm --prefix web ci` and `npm run build` to the integration-tests job
before `test:integration`, matching what the build job already does. Using
`needs: build` + artifact sharing would serialize the two jobs and undo the
parallelism PR #4093 was buying, so the build is duplicated intentionally.

.github/workflows/ci.yml

@@ -179,6 +179,17 @@ jobs:
       - name: Install dependencies
         run: npm ci
 
+      # Integration tests need the same compiled artifacts as the build job:
+      # - dist/loader.js and packages/pi-coding-agent/dist/** from `npm run build`
+      # - web/node_modules/.bin/next for tests that shell `build:web-host` at runtime
+      # Duplicating the build here (instead of sharing artifacts via needs: build)
+      # preserves wall-clock parallelism with the build job — see PR #4093.
+      - name: Install web host dependencies
+        run: npm --prefix web ci
+
+      - name: Build
+        run: npm run build
+
       - name: Run integration tests
         run: npm run test:integration