test: replace os.Args mutation with cobra command factory

[mendsec]

Context

The current integration tests in tests/integration_test.go mutate os.Args and os.Stdout globally. Even with the C6/C7 fixes in v0.2.0, the approach is fragile because Cobra's PersistentPreRun hooks and flag default values are set once at package init. The correct pattern is to expose a NewRootCmd() constructor in internal/cli that returns a fresh *cobra.Command per call, making each test fully isolated without process-level side effects.

Acceptance criteria

• internal/cli exports NewRootCmd() *cobra.Command
• All integration tests call NewRootCmd().ExecuteC() instead of the package-level cli.Execute()
• os.Args is never mutated in any test file
• go test -race -count=3 ./... passes deterministically
• TestMain go build step retained for subprocess signal test only