## Summary
This PR optimizes the CI build pipeline by enabling parallel workspace builds in all environments and removing redundant `posttest` hooks that caused unnecessary rebuilds after test execution.
## Changes
- **`scripts/build.js`**: Removed the sequential build bottleneck in CI. The script now always uses parallelized builds (ensuring `@google/gemini-cli-core` is built first), which significantly reduces build time in GitHub Actions.
- **`package.json`**, **`packages/cli/package.json`**, **`packages/core/package.json`**: Removed `"posttest": "npm run build"`. Since the CI workflow explicitly builds the project before running tests, these hooks were triggering redundant full rebuilds in every test shard, wasting significant CI minutes.
## Impact
- **Reduced CI Spend**: Eliminates approximately 15 redundant full rebuilds per push across the various test shards (Linux, Mac, Windows).
- **Faster Feedback**: Decreases total CI wall-clock time by parallelizing the initial build in the `lint` and `test` jobs.
Verified with a local full build and targeted unit tests. No regressions identified.
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