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).

install.ps1

@@ -30,4 +30,4 @@ if ($Link) {
 
 Write-Host ""
 Write-Host "Done. Restart Claude Code or run /reload."
-Write-Host "Skills auto-trigger. engineering-principles skill auto-loads at session start."
+Write-Host "Skills auto-trigger. SessionStart hook injects engineering-principles into context."