fix(hooks): restore Superpowers-style SessionStart command hook

Properly follow Superpowers 6.1.1 pattern:
- SessionStart command hook (type: command) runs run-hook.cmd session-start
- run-hook.cmd is a cross-platform polyglot wrapper (Unix/Windows)
- session-start script reads engineering-principles/SKILL.md and injects
  it as additionalContext wrapped in EXTREMELY_IMPORTANT tags
- The 29 engineering principles are active in context from turn 1

plugin.json: remove sessionStart.skill, keep skills: ./skills/
The command hook is the primary mechanism (just like Superpowers' using-superpowers).

hooks/hooks.json

@@ -1,3 +1,16 @@
 {
-  "hooks": {}
+  "hooks": {
+    "SessionStart": [
+      {
+        "matcher": "startup|clear|compact",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "\"${CLAUDE_PLUGIN_ROOT}/hooks/run-hook.cmd\" session-start",
+            "async": false
+          }
+        ]
+      }
+    ]
+  }
 }