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).

.claude-plugin/plugin.json

@@ -7,8 +7,5 @@
     "email": "asepharyana@users.noreply.github.com"
   },
   "keywords": ["best-practice", "clean-code", "engineering-guide", "programming-standards", "architecture"],
-  "skills": "./skills/",
-  "sessionStart": {
-    "skill": "engineering-principles"
-  }
+  "skills": "./skills/"
 }