Suppress the startup banner for --json (closes #106)

The banner is printed to stdout before cobra parses, and
bannerSuppressedInArgs recognised only --quiet, -q and --cron. So every
--json document was preceded by two banner lines and a blank one, and
`vaultik snapshot list --json | jq` failed. Passing opts.JSON as
extraQuiet did not help: that calls UI.SetQuiet in an fx OnStart hook,
long after Entry has printed.

The raw-argv scan is extended rather than the banner moved after
parsing. root.go documents that the banner must survive cobra rejecting
its arguments and --help, and no single post-parse location covers those
paths. The subcommand-versus-persistent distinction does not decide it:
--cron is already in the suppression list and is itself subcommand-only,
existing on snapshot create alone, so this adds another instance of an
accepted imprecision rather than a new kind. The error directions are
asymmetric - a false positive loses a decorative banner, a false negative
corrupts a document - so the scan errs toward suppression, which is also
why --json=false suppresses, exactly as --quiet=false already does.

Four of the five --json commands now pipe into jq cleanly with no other
flags: snapshot list, snapshot verify, snapshot remove, remote info.
prune does not, because pruneLocalSnapshots writes three prose lines to
stdout with no --json awareness. That reproduces identically before this
change and -q never suppressed it either, since printlnStdout and
stdoutf bypass v.UI entirely. Tracked as #108.

Also fixed: TTYHandler's human-readable byte formatting did not survive
grouping, because the key check compared against the bare attribute name
and a grouped record presents it qualified. AGENTS.md policy 9 keyed the
log format on stdout's TTY-ness, which #82 made false by moving the
logger to stderr; it now names the log stream. Vaultik.Stderr keeps its
field with the comment amended to say outright that nothing writes to
it, and the dead listEnv.stderr is removed.

internal/cli/entry.go

@@ -1,6 +1,7 @@
 package cli
 
 import (
+	"io"
 	"os"
 	"strings"
 	"time"
@@ -14,18 +15,12 @@ import (
 const shortCommitLen = 12
 
 // Entry is the main entry point for the CLI application.
-// It prints the startup banner (unless a quiet flag is present in os.Args),
-// executes the root cobra command, and routes any returned error through
-// the ui.Writer so the user sees a properly formatted "🛑 ERROR:" line.
+// It prints the startup banner to stdout (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.
 func Entry() {
-	if !bannerSuppressedInArgs(os.Args[1:]) {
-		short := globals.Commit
-		if len(short) > shortCommitLen {
-			short = short[:shortCommitLen]
-		}
-
-		writeStartupBanner(ui.New(os.Stdout), time.Now().UTC(), short)
-	}
+	emitStartupBanner(os.Args[1:], os.Stdout)
 
 	rootCmd := NewRootCommand()
 	rootCmd.SilenceErrors = true
@@ -37,6 +32,24 @@ func Entry() {
 	}
 }
 
+// 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.
+func emitStartupBanner(args []string, w io.Writer) {
+	if bannerSuppressedInArgs(args) {
+		return
+	}
+
+	short := globals.Commit
+	if len(short) > shortCommitLen {
+		short = short[:shortCommitLen]
+	}
+
+	writeStartupBanner(ui.New(w), time.Now().UTC(), short)
+}
+
 // ReportErrorf emits a user-facing error to stderr in the standard
 // 🛑 ERROR: format. Use it from goroutine error paths (where returning
 // an error to cobra isn't an option) and anywhere else a CLI command
@@ -46,9 +59,20 @@ func ReportErrorf(format string, args ...any) {
 }
 
 // bannerSuppressedInArgs reports whether any of args is a flag that
-// should suppress the startup banner (--quiet/-q/--cron). Stops at the
-// "--" argument terminator. Recognizes both long forms and short -q,
-// including combined short flags like "-qv".
+// should suppress the startup banner (--quiet/-q/--cron/--json). Stops
+// at the "--" argument terminator. Recognizes both long forms and short
+// -q, including combined short flags like "-qv".
+//
+// This scans the raw argument vector because the banner is printed
+// before cobra parses anything — deliberately, so that it still appears
+// when cobra rejects the arguments and on --help. The consequence is
+// that a flag is matched wherever it occurs in the vector, including
+// positions where the command it belongs to would not accept it.
+// --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.
 func bannerSuppressedInArgs(args []string) bool {
 	for _, a := range args {
 		if a == "--" {
@@ -56,11 +80,13 @@ func bannerSuppressedInArgs(args []string) bool {
 		}
 
 		switch a {
-		case "--quiet", "-q", "--cron":
+		case "--quiet", "-q", "--cron", "--json":
 			return true
 		}
 
-		if strings.HasPrefix(a, "--quiet=") || strings.HasPrefix(a, "--cron=") {
+		if strings.HasPrefix(a, "--quiet=") ||
+			strings.HasPrefix(a, "--cron=") ||
+			strings.HasPrefix(a, "--json=") {
 			return true
 		}
 		// Combined short flags like -qv or -vq.