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

hooks/hooks.json

@@ -2,12 +2,11 @@
   "hooks": {
     "SessionStart": [
       {
-        "matcher": "startup|resume|clear|compact",
+        "matcher": "",
         "hooks": [
           {
             "type": "command",
-            "command": "\"${CLAUDE_PLUGIN_ROOT}/hooks/session-start\"",
-            "async": false
+            "command": "node \"${CLAUDE_PLUGIN_ROOT}/hooks/session-start\""
           }
         ]
       }