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/run-hook.cmd

@@ -0,0 +1,41 @@
+: << 'CMDBLOCK'
+@echo off
+REM Cross-platform polyglot wrapper for hook scripts.
+REM On Windows: cmd.exe runs the batch portion, which finds and calls bash.
+REM On Unix: the shell interprets this as a script (: is a no-op in bash).
+REM
+REM Usage: run-hook.cmd <script-name> [args...]
+
+if "%~1"=="" (
+    echo run-hook.cmd: missing script name >&2
+    exit /b 1
+)
+
+set "HOOK_DIR=%~dp0"
+
+REM Try Git for Windows bash in standard locations
+if exist "C:\Program Files\Git\bin\bash.exe" (
+    "C:\Program Files\Git\bin\bash.exe" "%HOOK_DIR%%~1" %2 %3 %4 %5 %6 %7 %8 %9
+    exit /b %ERRORLEVEL%
+)
+if exist "C:\Program Files (x86)\Git\bin\bash.exe" (
+    "C:\Program Files (x86)\Git\bin\bash.exe" "%HOOK_DIR%%~1" %2 %3 %4 %5 %6 %7 %8 %9
+    exit /b %ERRORLEVEL%
+)
+
+REM Try bash on PATH
+where bash >nul 2>nul
+if %ERRORLEVEL% equ 0 (
+    bash "%HOOK_DIR%%~1" %2 %3 %4 %5 %6 %7 %8 %9
+    exit /b %ERRORLEVEL%
+)
+
+REM No bash found — exit silently
+exit /b 0
+CMDBLOCK
+
+# Unix: run the named script directly
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+SCRIPT_NAME="$1"
+shift
+exec bash "${SCRIPT_DIR}/${SCRIPT_NAME}" "$@"