Scaffold StepForge: docs, license, audit, npm project

- Environment audit and stack decision in build/agent_audit.md
  (Node + Electron shell over dependency-free core; deviation recorded)
- Rewrite README/ARCHITECTURE/CONTRIBUTING for StepForge
- Add SECURITY.md threat model, CODE_OF_CONDUCT.md, CHANGELOG.md, MPL-2.0
- Replace template grep checks with structural workflow check

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,7 @@
+node_modules/
+dist/
+build/artifacts/
+*.log
+.tmp/
+tests/.tmp/
+examples/.tmp/