fix: reduce install warning noise

[wbugitlab1]

Summary

• move the local Xenova embedding backend out of the default optional dependency install surface and make it an optional peer
• remove unused ONNX optional package entries from the package metadata/lockfile so default installs avoid sharp/prebuild-install warning paths
• update quality gates, README guidance, and runtime error messages for same-prefix optional embedding installs

Verification

• corepack pnpm exec vitest run test/quality-gates.test.ts test/embedding-provider.test.ts test/reranker.test.ts --exclude test/integration.test.ts
• corepack pnpm run build
• npm pack --dry-run --json
• local tarball consumer install smoke: no Anthropic, Xenova, ONNX, sharp, prebuild-install, node-domexception entries or issue 安装时有一些警告，虽然不影响使用，但我觉得对新用户体验很差 #258 warning signatures
• corepack pnpm run lint
• osv-scanner scan source .
• semgrep scan --config p/default --error --metrics=off .
• gitleaks protect --staged --redact
• git diff --check

Closes #258