Add --long-description / --long-description-file to `hermes slack
manifest` so the generated app manifest can carry Slack's
display_information.long_description (175–4,000 characters), with
validation of the length bounds, mutual-exclusion with --slashes-only,
and UTF-8 file input. Also propagate the manifest command's exit status
through cmd_slack so validation failures reach the shell.
Squash of the two commits from PR #65256 — one commit per contributor
on this salvage branch.
Salvaged from #65256
0a5d8a16fc