chore: consolidate plugin for sharing

- Add .gitignore (ignore .archeflow/ runtime state)
- Move custom archetype examples from .archeflow/ to examples/
- Remove onboarding skill (covered by using-archeflow + README)
- Remove internal planning doc
- Clean roadmap (remove tool.archeflow references)
- Simplify session-start hook config
- Polish README for external users

.gitignore

@@ -0,0 +1,10 @@
+# Runtime state (created per-project, not part of plugin)
+.archeflow/
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Editor
+*.swp
+*~