chore: document Graphify code-graph setup, ignore its output dir

Records the AST-derived code-graph tooling (query/explain/affected commands
against graphify-out/graph.json) so future sessions know it's available
and why it's not wired in as a global skill/hook.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Xsxk4ZkDQVVaLUcixDcqb5

.gitignore

@@ -48,3 +48,6 @@ app.*.map.json
 **/node_modules/
 **/.next/
 **/dist/
+
+# Graphify code knowledge graph (regenerated locally via git hooks)
+graphify-out/