cli: errors from RunE are silently swallowed (no stderr)

[zeevdr]

Description

cmd/decree/main.go sets SilenceErrors: true and SilenceUsage: true on the root cobra command, but main() only calls os.Exit(1) on error without printing it. Any error returned from a command's RunE (e.g. an invalid --file schema passed to decree docgen --file) is swallowed entirely — the CLI exits 1 with no stdout/stderr output, making failures impossible to diagnose.

Acceptance criteria

• main() prints the returned error (e.g. to stderr) before os.Exit(1)
• Verify with a failing offline command (e.g. decree docgen --file <invalid-schema.yaml>) that the error message is now printed
• Add a test covering this error-printing path (e.g. in cmd/decree/cli_test.go or helpers_test.go)
• Run go tool cover -func to confirm new code is covered per repo coverage policy