Add startup persistence for support sessions

- Added startup.rs module for Windows registry operations
- Agent adds itself to HKCU\...\Run on session start
- Agent removes itself from startup on session end
- Works with user-level registry (no admin required)
- Added Win32_System_Registry feature to windows crate

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

agent/Cargo.toml

@@ -71,6 +71,7 @@ windows = { version = "0.58", features = [
     "Win32_UI_WindowsAndMessaging",
     "Win32_System_LibraryLoader",
     "Win32_System_Threading",
+    "Win32_System_Registry",
     "Win32_Security",
 ]}