docs: update repository references from glittercowboy to gsd-build

The repository was transferred from the glittercowboy org to gsd-build,
but several files still referenced the old org in URLs. This updates all
repository URL references across READMEs (all languages), package.json,
and the update workflow. Also removes a duplicate language selector in
the main README header.

Files intentionally unchanged:
- CHANGELOG.md (historical entries)
- CODEOWNERS, FUNDING.yml, SECURITY.md (reference @glittercowboy as a
  GitHub username/handle, not a repo URL)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -29,11 +29,11 @@
   "license": "MIT",
   "repository": {
     "type": "git",
-    "url": "git+https://github.com/glittercowboy/get-shit-done.git"
+    "url": "git+https://github.com/gsd-build/get-shit-done.git"
   },
-  "homepage": "https://github.com/glittercowboy/get-shit-done",
+  "homepage": "https://github.com/gsd-build/get-shit-done",
   "bugs": {
-    "url": "https://github.com/glittercowboy/get-shit-done/issues"
+    "url": "https://github.com/gsd-build/get-shit-done/issues"
   },
   "engines": {
     "node": ">=20.0.0"