docs: align public release lag wording

[xiaojiou176]

Summary

• align the public front-door truth surfaces around one release boundary: v0.1.0-alpha.3 is still the latest published prerelease, but main has moved ahead of that tag
• keep the machine-readable manifest, README, distribution contract, Pages mirror, storefront demo-status notes, and GitHub storefront manual steps saying the same thing
• avoid changing historical release receipts such as docs/releases/v0.1.0-alpha.3.md; this PR only updates current-truth surfaces

Test Plan

• git diff --check
• python3 -m json.tool configs/mcp_public_manifest.json >/dev/null
• npm run docs:check
• negative search for stale current-truth wording outside historical release notes

Breaking Changes

None

Rollout / Risk

• 风险等级: Low
• 灰度策略: 直接发布
• 回滚方案: revert this PR to restore the previous storefront wording
• 监控指标: PR required checks and front-door docs consistency

Observability

• 新增日志: None
• 新增指标: None
• 告警变更: None

Checklist

• Self-reviewed code
• No debug/print statements left
• No TODO without issue reference
• Documentation updated (if needed)
• Tests added/updated