fix(02): write fixtures exclusively and only on successful flush (WR-01)

Co-authored-by: Cursor <cursoragent@cursor.com>

cmd/summer/parity.go

@@ -21,6 +21,7 @@ func parityProxyCommand() bonfire.Command {
 			{Name: "rules", Description: "Committed YAML capture rules"},
 			{Name: "vars", Description: "Private mode-0600 variable store outside fixtures"},
 			{Name: "fixtures", Description: "Directory for nuxt/ and mcp/ session flows"},
+			{Name: "update", Description: "Overwrite existing session fixtures (true/false)"},
 		},
 		Run: runParityProxy,
 	}
@@ -88,6 +89,7 @@ func runParityProxy(ctx context.Context, in bonfire.Input, out bonfire.Output) e
 		Rules:    rules,
 		VarsPath: varsPath,
 		Fixtures: fixtures,
+		Update:   flagBool(in, "update"),
 	})
 	if err != nil {
 		return err