chore: bump version and changelog (v1.12.1.0)

Also updates test/gstack-brain-init-gh-mock.test.ts to match the fixed
behavior of bin/gstack-brain-init (the assertion previously required
`--source`, which was the bug being fixed in 04185d8f).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "gstack",
-  "version": "1.12.0.0",
+  "version": "1.12.1.0",
   "description": "Garry's Stack — Claude Code skills + fast headless browser. One repo, one install, entire AI engineering workflow.",
   "license": "MIT",
   "type": "module",