cli-v1.5.1

Add horizontal-rule stripping; bump app+CLI to 1.5.1

Adds a new `horizontalRule` sugar to SugarCore (CommonMark thematic
breaks `---`/`***`/`___` in plain text, `<hr>` in HTML) with the matching
CLI flag (`--horizontal-rules`) and app checklist entry (via Sugar.allCases).
Off by default like the other non-default sugars; emphasis markers are
unaffected since they always carry inner content.

Aligns the two release tracks on a single version number going forward:
app 1.5.0 and CLI 1.4.0 both move to 1.5.1. Docs (CLAUDE.md, RELEASING.md)
updated to record the lockstep-versioning policy.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>