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",

packages/cli/package.json

@@ -20,7 +20,6 @@
     "format": "prettier --write .",
     "test": "vitest run",
     "test:ci": "vitest run",
-    "posttest": "npm run build",
     "typecheck": "tsc --noEmit"
   },
   "files": [

packages/core/package.json

@@ -16,7 +16,6 @@
     "format": "prettier --write .",
     "test": "vitest run",
     "test:ci": "vitest run",
-    "posttest": "npm run build",
     "typecheck": "tsc --noEmit"
   },
   "files": [

scripts/build.js

@@ -33,31 +33,26 @@ if (!existsSync(join(root, 'node_modules'))) {
 // build all workspaces/packages
 execSync('npm run generate', { stdio: 'inherit', cwd: root });
 
-if (process.env.CI) {
-  console.log('CI environment detected. Building workspaces sequentially...');
-  execSync('npm run build --workspaces', { stdio: 'inherit', cwd: root });
-} else {
-  // Build core first because everyone depends on it
-  console.log('Building @google/gemini-cli-core...');
-  execSync('npm run build -w @google/gemini-cli-core', {
-    stdio: 'inherit',
-    cwd: root,
-  });
+// Build core first because everyone depends on it
+console.log('Building @google/gemini-cli-core...');
+execSync('npm run build -w @google/gemini-cli-core', {
+  stdio: 'inherit',
+  cwd: root,
+});
 
-  // Build the rest in parallel
-  console.log('Building other workspaces in parallel...');
-  const workspaceInfo = JSON.parse(
-    execSync('npm query .workspace --json', { cwd: root, encoding: 'utf-8' }),
-  );
-  const parallelWorkspaces = workspaceInfo
-    .map((w) => w.name)
-    .filter((name) => name !== '@google/gemini-cli-core');
+// Build the rest in parallel
+console.log('Building other workspaces in parallel...');
+const workspaceInfo = JSON.parse(
+  execSync('npm query .workspace --json', { cwd: root, encoding: 'utf-8' }),
+);
+const parallelWorkspaces = workspaceInfo
+  .map((w) => w.name)
+  .filter((name) => name !== '@google/gemini-cli-core');
 
-  execSync(
-    `npx npm-run-all --parallel ${parallelWorkspaces.map((w) => `"build -w ${w}"`).join(' ')}`,
-    { stdio: 'inherit', cwd: root },
-  );
-}
+execSync(
+  `npx npm-run-all --parallel ${parallelWorkspaces.map((w) => `"build -w ${w}"`).join(' ')}`,
+  { stdio: 'inherit', cwd: root },
+);
 
 // also build container image if sandboxing is enabled
 // skip (-s) npm install + build since we did that above