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-targets/claude-home.js

@@ -39,6 +39,10 @@ function getClaudeManagedDestinationPath(adapter, sourceRelativePath, input) {
     );
   }
 
+  if (normalizedSourcePath === 'docs' || normalizedSourcePath.startsWith('docs/')) {
+    return path.join(targetRoot, normalizedSourcePath);
+  }
+
   return null;
 }