prune,ui,docs: color the digest, add -f/--force and a sizing spinner

- -f/--force skips the size pass and the confirmation prompt: remove
  immediately, reporting counts but no sizes. The fastest cleanup
  path; --dry-run still previews.
- The default path shows the grouped digest and asks before removing
  (the raw-mode yes/no prompt; non-interactive sessions keep their
  headless path, so scripts reach for --force).
- Sizes are computed under a spinner ("Computing sizes of
  directories to prune..."), and the digest colors the counts (bold)
  and the sizes (cyan) on terminals only.

README.md

@@ -188,7 +188,8 @@ the oldest degrades to a metadata-only record (manifest and log stay),
 and `pkh prune` removes records older than a week. `pkh prune --dry-run`
 previews everything it would remove, grouped by kind with sizes;
 `--all` ignores the age and removes everything a live build does not
-hold, recent records and staged trees included. `pkh deb
+hold, recent records and staged trees included; `-f/--force` skips the
+size pass and the confirmation prompt, removing immediately. `pkh deb
 --incremental` never forks a record built for a different
 series/architecture.