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/cursor_test.go

@@ -342,7 +342,7 @@ func TestCursorCreatesMissingGuardAndRejectsUnmanagedRuntimePaths(t *testing.T)
 				fixture.config.HookScript = unmanaged
 			}
 			if err := fixture.plugin.Install(fixture.config); err == nil ||
-				!strings.Contains(err.Error(), "managed agent directory") {
+				!strings.Contains(err.Error(), "managed Elydora agent directory") {
 				t.Fatalf("unmanaged %s error = %v", field, err)
 			}
 			if _, err := os.Stat(fixture.configPath); !os.IsNotExist(err) {