feat(installer): add --locale flag for translated docs installation

Adds `--locale <code>` support to the ECC installer so users can install
localized reference docs (agents, commands, skills, rules) into
`~/.claude/docs/<locale>/` alongside the existing English installation.

Changes:
- manifests/install-modules.json: add 8 locale doc modules (docs-ja-JP,
  docs-zh-CN, docs-ko-KR, docs-pt-BR, docs-ru, docs-tr, docs-vi-VN,
  docs-zh-TW), each with kind="docs" and defaultInstall=false
- manifests/install-components.json: add 8 locale: components mapping to
  the new modules
- scripts/lib/install-manifests.js: add locale: family prefix,
  SUPPORTED_LOCALES, LOCALE_ALIAS_TO_COMPONENT_ID (with aliases like
  ja=ja-JP, zh=zh-CN, ko=ko-KR), and listSupportedLocales()
- scripts/lib/install/request.js: add --locale flag to parseInstallArgs(),
  resolve locale alias → component ID in normalizeInstallRequest(), throw
  on unsupported locale codes
- scripts/lib/install-targets/claude-home.js: map docs/<locale>/ source
  paths to ~/.claude/docs/<locale>/ destination (side-by-side, no overwrite
  of English files)
- scripts/install-apply.js: import listSupportedLocales, add --locale
  usage line and available locales list to --help output

Usage examples:
  ./install.sh --locale ja                    # Japanese docs only
  ./install.sh --profile core --locale zh-CN  # core profile + zh-CN docs
  ./install.sh typescript --locale ja         # legacy + locale (errors)

scripts/lib/install/request.js

@@ -1,6 +1,6 @@
 'use strict';
 
-const { validateInstallModuleIds } = require('../install-manifests');
+const { validateInstallModuleIds, LOCALE_ALIAS_TO_COMPONENT_ID, listSupportedLocales } = require('../install-manifests');
 
 const LEGACY_INSTALL_TARGETS = ['claude', 'cursor', 'antigravity'];
 
@@ -27,6 +27,7 @@ function parseInstallArgs(argv) {
     includeComponentIds: [],
     excludeComponentIds: [],
     languages: [],
+    locale: null,
   };
 
   for (let index = 0; index < args.length; index += 1) {
@@ -60,6 +61,9 @@ function parseInstallArgs(argv) {
         parsed.excludeComponentIds.push(componentId.trim());
       }
       index += 1;
+    } else if (arg === '--locale') {
+      parsed.locale = args[index + 1] || null;
+      index += 1;
     } else if (arg === '--dry-run') {
       parsed.dryRun = true;
     } else if (arg === '--json') {
@@ -84,9 +88,17 @@ function normalizeInstallRequest(options = {}) {
   const moduleIds = validateInstallModuleIds(
     dedupeStrings([...(config?.moduleIds || []), ...(options.moduleIds || [])])
   );
+  const locale = options.locale || config?.locale || null;
+  const localeComponentId = locale ? LOCALE_ALIAS_TO_COMPONENT_ID[locale] : null;
+  if (locale && !localeComponentId) {
+    throw new Error(
+      `Unsupported locale: "${locale}". Supported locales: ${listSupportedLocales().join(', ')}`
+    );
+  }
   const includeComponentIds = dedupeStrings([
     ...(config?.includeComponentIds || []),
     ...(options.includeComponentIds || []),
+    ...(localeComponentId ? [localeComponentId] : []),
   ]);
   const excludeComponentIds = dedupeStrings([
     ...(config?.excludeComponentIds || []),
@@ -102,7 +114,7 @@ function normalizeInstallRequest(options = {}) {
 
   if (usingManifestMode && legacyLanguages.length > 0) {
     throw new Error(
-      'Legacy language arguments cannot be combined with --profile, --modules, --with, --without, or manifest config selections'
+      'Legacy language arguments cannot be combined with --profile, --modules, --with, --without, --locale, or manifest config selections'
     );
   }