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_banner_test.go

@@ -81,7 +81,7 @@ var jsonArgumentVectors = map[string][]string{
 // internal/vaultik's TestListSnapshots_JSONStdoutIsOnlyTheDocument
 // guards the same contract one layer down, but it calls the library
 // function directly and so cannot see Entry, which is where the
-// contamination was: the startup banner is written to stdout before
+// contamination was: the startup banner was written to stdout before
 // cobra parses anything, and the suppression scan did not know about
 // --json. The two banner lines and the blank line landed ahead of the
 // document and `jq` refused the result.
@@ -247,9 +247,8 @@ func TestEntryJSONStdoutIsExactlyOneDocument(t *testing.T) {
 // captureProcessStdout redirects the process's own stdout to a pipe for
 // the duration of fn and returns what was written to it. The redirection
 // has to be at the file-descriptor level rather than through an injected
-// writer, because the banner and the JSON encoder reach os.Stdout
-// independently and the point of the test is that both land in the same
-// place.
+// writer, because Entry and the commands it runs each reach os.Stdout
+// directly.
 //
 // Not parallel-safe: os.Stdout is process-global.
 func captureProcessStdout(t *testing.T, fn func()) string {