Changing a single color kept wrecking the rest because the agent hand-authored a
new skin (often from `default`, which has no `background`, resetting the terminal
to black). Add `hermes skin set <key> <hex>`: edits the ACTIVE skin's one key in
place (a built-in is forked into an editable copy carrying its full palette), so
everything else — background included — is preserved. Plus `skin use` / `skin
list`. The skill now points tweaks at this command instead of hand-authoring.
