Keep command output to the README's stdout and stderr rules (closes #224)

The startup banner moves from stdout to stderr, so a `completion`
script, a `config get` value and the hidden `__complete` command print
only their own output. `--quiet`, `--cron` and `--json` still suppress
it.

A failing `remote info`, `prune` or `snapshot remove` under `--json`
now reports its error on stderr. Their reporters returned early under
`--json`, so the failure reached neither stream.

`snapshot verify --quiet` writes no report. A failure is still
returned and printed on stderr, with the same exit status.

Judgement call: the banner's stream, posted on the issue for the owner.

Model: opus-5-5

internal/cli/entry.go

@@ -16,16 +16,18 @@ import (
 const shortCommitLen = 12
 
 // Entry is the main entry point for the CLI application.
-// It prints the startup banner to stdout (unless a banner-suppressing
+// It prints the startup banner to stderr (unless a banner-suppressing
 // flag is present in os.Args — see bannerSuppressedInArgs), executes the
 // root cobra command, and routes any returned error through the
 // ui.Writer so the user sees a properly formatted "🛑 ERROR:" line.
+// The banner goes to stderr because stdout carries only the output the
+// user asked for, such as a completion script or a `config get` value.
 //
 // It returns the process exit code (0 on success, 1 on error) rather
 // than calling os.Exit, so that main's deferred profile writers run
 // before the process ends. See run in cmd/vaultik/main.go.
 func Entry() int {
-	emitStartupBanner(os.Args[1:], os.Stdout)
+	emitStartupBanner(os.Args[1:], os.Stderr)
 
 	rootCmd := NewRootCommand()
 	rootCmd.SilenceErrors = true
@@ -33,8 +35,9 @@ func Entry() int {
 	err := rootCmd.Execute()
 	if err != nil {
 		// An operation that ran inside the fx app has already reported
-		// its own failure (and suppressed it under --json); errReported
-		// says so. Printing it again here would double the error line.
+		// its own failure (`snapshot verify --json` puts it in the
+		// document instead); errReported says so. Printing it again
+		// here would double the error line.
 		// Every other error — bad arguments, a config that would not
 		// load — reaches Entry unreported, so it is shown here.
 		if !errors.Is(err, errReported) {
@@ -49,9 +52,8 @@ func Entry() int {
 
 // emitStartupBanner writes the startup banner to w unless args (the
 // argument vector with the program name already stripped) contains a
-// flag that suppresses it. Split out of Entry so that the decision — the
-// only thing standing between a --json invocation and a parseable
-// stdout — is reachable from a test without running the whole CLI.
+// flag that suppresses it. Split out of Entry so that the decision is
+// reachable from a test without running the whole CLI.
 func emitStartupBanner(args []string, w io.Writer) {
 	if bannerSuppressedInArgs(args) {
 		return
@@ -86,8 +88,7 @@ func ReportErrorf(format string, args ...any) {
 // --json is a subcommand flag rather than a persistent one, but so is
 // --cron (it exists only on `snapshot create`), so this adds no new
 // class of imprecision. The only cost of a false positive is a missing
-// decorative banner; the cost of a false negative is a corrupt document
-// on stdout, so the scan errs deliberately in that direction.
+// decorative banner.
 func bannerSuppressedInArgs(args []string) bool {
 	for _, a := range args {
 		if a == "--" {