feat(xueqiu): add comments command

[Astro-Han]

Description

Add a new xueqiu comments <symbol> command for stock-scoped discussion posts.

This PR adds a TypeScript adapter that:

• fetches stock discussion posts with browser session cookies
• supports --limit with a clear 1..100 contract
• normalizes output fields for downstream JSON/table usage
• handles auth/challenge failures, empty pages, duplicate rows, and malformed items more defensively

Closes #576

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🌐 New site adapter
• 📝 Documentation
• ♻️ Refactor
• 🔧 CI / build / tooling

Checklist

• I ran the checks relevant to this PR
• I updated tests or docs if needed
• I included output or screenshots when useful

Documentation (if adding/modifying an adapter)

• Added doc page under docs/adapters/ (if new adapter)
• Updated docs/adapters/index.md table (if new adapter)
• Updated sidebar in docs/.vitepress/config.mts (if new adapter)
• Updated README.md / README.zh-CN.md when command discoverability changed
• Used positional args for the command's primary subject unless a named flag is clearly better
• Normalized expected adapter failures to CliError subclasses instead of raw Error

Screenshots / Output

Checks run:

• npx vitest run src/clis/xueqiu/comments.test.ts
• npm run typecheck
• npm run test:adapter