docs: clarify AI SRE beta access and billing

[ysyneu]

Summary

• Update AI SRE private-beta notices to link directly to the Feishu application form.
• Clarify that Pro or higher accounts can apply for free beta access and are whitelisted after approval.
• Clarify that AI SRE becomes paid after commercialization, with billing information shared before users choose to activate or pause usage.
• Add .worktrees/ to .gitignore for repo-local worktrees.

Validation

• git diff --check
• PATH="$HOME/.nvm/versions/node/v22.12.0/bin:$PATH" mint broken-links