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.

AGENTS.md

@@ -83,8 +83,8 @@ Version: 2025-06-08
    possible to mock or stub these side-effects in tests.
 
 9. Always use structured logging.  Log any relevant state/context with the
-   messages (but do not log secrets).  If stdout is not a terminal, output
-   the structured logs in jsonl format.
+   messages (but do not log secrets).  If the log stream is not a terminal,
+   output the structured logs in jsonl format.
 
 10. Avoid using bare strings or numbers in code, especially if they appear
     anywhere more than once.  Always define a constant (usually at the top