Slack has built-in slash commands (e.g. /status, /me, /join) that apps
cannot register. When running `hermes slack manifest --write`, the
generated manifest included /status, causing Slack to reject the entire
manifest with a reserved-command error.
Add _SLACK_RESERVED_COMMANDS frozenset of all known Slack built-ins and
skip them in slack_native_slashes(). Affected commands remain reachable
via /hermes <command>.
Tests updated:
- New test_excludes_slack_reserved_commands validates no leaks
- test_includes_canonical_commands no longer asserts /status
- test_telegram_parity accounts for expected Slack-only exclusions
a717199bbf