`hermes config set gateway.discord.gateway_restart_notification true` wrote the
typo into config.yaml and only then printed the "not a recognized config key — it
was saved anyway" notice (#112003). Under a KNOWN section an unknown sub-key can
only be a typo, so `set_config_value` now exits non-zero via `_exit_invalid`
before reading or writing config.yaml, with the did-you-mean hint.
Scope preserved from ed3a0b3 (warn-after-write): unknown TOP-LEVEL keys are
still written with the post-write notice, because top-level scalars are bridged
into os.environ for skills/external apps and that namespace is open by design;
the `_OPEN_SUBKEY_TOP_LEVEL_KEYS` / platform-container exemptions in
`_validate_config_key` are untouched, and `--force` keeps writing anything. This
is the fail-fast piece the maintainer scoped in the close comment on #111133.
`_validate_config_key` also suggests the path minus its wrong prefix
(`gateway.discord.x` -> `discord.x`) when no same-level sibling is close; the
headline typo previously produced no hint at all.
Docs: cli-commands.md `set`/`unset` rows, configuration.md tip, `--force` help.
Co-authored-by: KoNit-K <124019182+KoNit-K@users.noreply.github.com>
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