docs: update READMEs to 13 engines + GitHub description

feat: v0.19.0 — 13 engines

docs: complete README rewrite — 8 engines, built-in reference, real output examples

feat: v0.18.0 — 8 engines, built-in downgraded, VirusTotal positioning

refactor: downgrade built-in engine to reference-only

fix: Bearer auto-install + README 8 engines + v0.17.1

fix: auto-install build error — escape regex in shell command

chore: release v0.17.0 — 8 engines

feat: expand to 8 engines — Trivy, Gitleaks, Bandit, Bearer

docs: add tagline — One scan. Four engines. One report.

docs: new positioning — we aggregate, not compete

docs: remove unnecessary VirusTotal line from zh README

docs: English README + Chinese README, both warm tone

docs: README rewrite — warm tone, conversational Chinese

feat: default to multi-engine, remove --engines flag