chore: add changeset for BaseAdapter extraction

Author: FL4TLiN3

.changeset/extract-base-adapter.md

@@ -0,0 +1,12 @@
+---
+"@perstack/core": patch
+"@perstack/docker": patch
+"@perstack/claude-code": patch
+"@perstack/cursor": patch
+"@perstack/gemini": patch
+"@perstack/runtime": patch
+---
+
+refactor: extract BaseAdapter from @perstack/core to @perstack/adapter-base
+
+This makes @perstack/core client-safe by removing child_process dependency.