Refuse partially staged files in sley ready --fix

[cgraf78]

Summary

Make the sley ready --fix git path refuse partially staged files, so
the commit gate shares one formatting-safety policy with the standalone
sley fix command.

sley ready --fix --commit is the real commit gate — the Git/Sapling
commit hook runs it on every commit-producing command. Its git --fix
path formatted each selected file in place with no guard against a
partially staged file (one with both staged and unstaged hunks).
Formatting rewrites the whole worktree file, folding the formatter's
output into the unstaged hunk — the exact corruption sley fix already
refuses via _sley_git_staged_partial_files. The two paths had
divergent, unsafe-vs-safe policy.

• add the guard to the git --fix branch, gated on the staged/commit
  scope (_SLEY_SCOPE_CHANGE == staged) — the only scope with an index
  to corrupt; plain ready --fix and non-git repos are unaffected, matching
  sley fix
• filter the selected files through _repo_existing_regular_files before
  the partial check, exactly as sley fix does, so symlinks and
  non-regular files (which the format loop skips anyway) can't trigger a
  spurious refusal
• on a partial file, print the same "refusing files with staged and
  unstaged changes" message, drop the selected-files cache, and return 2,
  byte-for-byte matching sley fix
• refusing partials first also makes the existing "formatter modified
  staged files" report accurate: every remaining file then has worktree
  content equal to its index, so the loop's worktree-hash comparison no
  longer mis-handles a file whose worktree differs from the index

Testing

• new sley-behavior-test cases: ready --fix --commit refuses a single
  partial file (exit 2, formats nothing, worktree unchanged); refuses a
  mixed batch where any file is partial (refuse-all parity, clean file
  left unformatted); and still formats a fully-staged clean file (exit 1
  with the re-stage report) so the guard does not over-refuse
• verified red-green: neutering the scope gate fails the refusal cases
  (formatter runs, worktree corrupted) while the clean-file case stays
  green
• full suite green (./test/sley-test: ok); checkrun lint and
  shellcheck -x lib/sley/ready.sh clean