fix(installer): write error and help text to stderr for consistent stream output

Extracted help text into getHelpText() and write both the error message
and usage help to stderr via process.stderr.write(). This ensures that
when output is redirected (e.g. 2>errors.txt), both the error and the
guidance appear in the same stream.

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

scripts/install-apply.js

@@ -17,10 +17,10 @@ const {
   parseInstallArgs,
 } = require('./lib/install/request');
 
-function showHelp(exitCode = 0) {
+function getHelpText() {
   const languages = listLegacyCompatibilityLanguages();
 
-  console.log(`
+  return `
 Usage: install.sh [--target <${LEGACY_INSTALL_TARGETS.join('|')}>] [--dry-run] [--json] <language> [<language> ...]
        install.sh [--target <${SUPPORTED_INSTALL_TARGETS.join('|')}>] [--dry-run] [--json] --profile <name> [--with <component>]... [--without <component>]...
        install.sh [--target <${SUPPORTED_INSTALL_TARGETS.join('|')}>] [--dry-run] [--json] --modules <id,id,...> [--with <component>]... [--without <component>]...
@@ -44,8 +44,11 @@ Options:
 
 Available languages:
 ${languages.map(language => `  - ${language}`).join('\n')}
-`);
+`;
+}
 
+function showHelp(exitCode = 0) {
+  console.log(getHelpText());
   process.exit(exitCode);
 }
 
@@ -139,8 +142,8 @@ function main() {
       printHumanPlan(result, false);
     }
   } catch (error) {
-    console.error(`Error: ${error.message}\n`);
-    showHelp(1);
+    process.stderr.write(`Error: ${error.message}\n\n${getHelpText()}`);
+    process.exit(1);
   }
 }