chore: split .stignore into shared file for auto-sync

Move ignore patterns to .stignore-shared (syncs across machines).
.stignore becomes a per-machine stub with #include directive.

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

.gitignore

@@ -1,6 +1,9 @@
 # Claude Code
 .claude/
 
+# Syncthing local stub (each machine has its own; real patterns in .stignore-shared)
+.stignore
+
 # Dependencies
 node_modules/
 jspm_packages/