docs(i18n): add German (de-DE) localization scout

Adds the German locale per maintainer guidance in issue #1980.

- docs/de-DE/README.md: full translation of the English root README
- docs/de-DE/GLOSSARY.md: German terminology decisions (which ECC
  terms stay English vs. German)
- manifests: docs-de-de module + locale:de-de component, modelled on
  the existing docs-zh-tw / locale:zh-tw entries
- scripts/lib/install-manifests.js: de-DE in SUPPORTED_LOCALES, plus
  de-DE and de aliases so `--locale de` and `--locale de-DE` resolve
- tests/lib/locale-install.test.js: focused de-DE coverage
- Deutsch added to the language selector tables in all 10 existing
  READMEs (root + 9 localized)

Refs #1980

manifests/install-components.json

@@ -589,6 +589,14 @@
       "modules": [
         "docs-zh-tw"
       ]
+    },
+    {
+      "id": "locale:de-de",
+      "family": "locale",
+      "description": "German (de-DE) translated reference docs installed to ~/.claude/docs/de-DE/.",
+      "modules": [
+        "docs-de-de"
+      ]
     }
   ]
 }