perf(ci): optimize CI build efficiency and remove redundant builds

This PR optimizes the CI/CD pipeline by enabling parallel workspace builds in CI and removing redundant `posttest` scripts that triggered unnecessary builds after testing.

### Changes:
- **Enable Parallel CI Builds**: Updated `scripts/build.js` to use the same parallelization logic in CI as it does for local development. This leverages multi-core runners more effectively.
- **Remove Redundant Builds**: Stripped `posttest: npm run build` from the root `package.json`, `packages/cli/package.json`, and `packages/core/package.json`. These packages were triggering full rebuilds after every test shard, even though the CI workflow explicitly runs a build step before testing.

### Impact:
- **Reduced CI Runtime**: Significant reduction in total Actions minutes by eliminating multiple redundant build cycles.
- **Lower Costs**: Expected to decrease GitHub Actions spend (recent metrics showed a +109% spike in spend, partly due to these inefficiencies).
- **Faster Feedback**: PRs will complete validation faster, improving developer productivity.

package.json

@@ -44,7 +44,6 @@
     "test:ci": "npm run test:ci --workspaces --if-present && npm run test:scripts && npm run test:sea-launch",
     "test:scripts": "vitest run --config ./scripts/tests/vitest.config.ts",
     "test:sea-launch": "vitest run sea/sea-launch.test.js",
-    "posttest": "npm run build",
     "test:always_passing_evals": "vitest run --config evals/vitest.config.ts",
     "test:all_evals": "cross-env RUN_EVALS=1 vitest run --config evals/vitest.config.ts",
     "test:e2e": "cross-env VERBOSE=true KEEP_OUTPUT=true npm run test:integration:sandbox:none",