fix(powershell): add timeout + shared self-healing instance

[cyytmd]

What

Replaces the per-command new PowerShell() + ps.dispose() pattern with a shared, self-healing PowerShellManager singleton.

Why

The original code creates and destroys a powershell.exe child process on every tool call (~0.5 s each) and never sets invocationTimeout on node-powershell. A command that hangs the shell (e.g. Format-Table -AutoSize probing terminal width in a non-interactive stdio pipe) freezes the MCP server forever — the 30 s client timeout fires, but the server-side promise never resolves, poisoning every subsequent call.

Changes

What	How
src/utils/powershell-manager.js	New. Shared singleton: reuses one powershell.exe, enforces 25 s invocation timeout (child-shell built-in), rebuilds the instance after timeout/fatal error, truncates output at 100 KB, serializes access with a lock, tears down cleanly on SIGINT/SIGTERM.
src/tools/powershell-tools.js	Removes createPowerShellInstance() + per-call try/catch/dispose. Delegates to psManager.execute().
src/tools/system-tools.js	Same — thin wrapper over psManager.execute().
src/tools/file-tools.js	Same — thin wrapper over psManager.execute().
src/server.js	Imports psManager; registers SIGINT/SIGTERM handlers for graceful teardown.

Tested

• ✅ Write-Output / Get-Date — instant return (singleton, no spawn overhead).
• ✅ Get-Process | Select ... | Format-Table -AutoSize — the exact command that previously froze the server indefinitely now returns within 1 s.
• ✅ Repeated calls — no leaked processes, no stream corruption.