feat: zero-dependency readability CLI (v0.2.0)

Add a `readlevel` CLI: a readability report for a file or stdin (Flesch, FK
grade, Gunning Fog, SMOG, Coleman–Liau, ARI), with --json, --keywords, --long,
and --max-grade <n> (exit 1 when the text is harder than grade n — a CI gate
for docs/marketing copy).

@types/node dev-only dep; resolveJsonModule enabled.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

feat: initial release of readlevel — local readability analysis (Fles…

…ch, grade level, reading time)