chore(go): audit providers claudecode codex cursor gemini

Add managed_runtime.go (paths, config validation, identity, runtime file
changes, status checks) and shell_command.go (quoting, PowerShell
encoding, command parsing) and move the four providers onto them.

Claude-Session: https://claude.ai/code/session_01Agoc9bRVE9KL7mmWtey9Vq

cmd/elydora/plugins/letta.go

@@ -219,4 +219,3 @@ func (p *LettaPlugin) Status() (PluginStatus, error) {
 	status.Installed = status.HookConfigured && status.HookScriptExists
 	return status, nil
 }
-