refactor(hooks): use sessionStart.skill instead of command hooks

- Replace SessionStart command hook with plugin.json's native
  sessionStart.skill field (like Superpowers' using-superpowers pattern)
- engineering-principles skill auto-loads at every session start
- Remove hooks/session-start, hooks/run-hook.cmd scripts
- Remove obsolete setup-hooks.sh (references deleted scripts)
- Empty hooks.json — hooks not needed, skill auto-load handles it

hooks/run-hook.cmd

@@ -1,44 +0,0 @@
-: << '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 Hook scripts use extensionless filenames so Claude Code's Windows
-REM auto-detection doesn't interfere.
-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}/${SCRIPT_NAME}" "$@"