docs: add sqlcmd --query flag gotcha

[Hinne1]

Summary

• Documents that multi-line SQL with leading -- comments breaks when passed as a positional argument to sqlcmd query
• Recommends using the --query named flag as a workaround

Discovered during NX-15422 when sqlcmd query "$(cat file.sql)" failed because the leading SQL comments were parsed as CLI flags.

Test plan

• Run npm start and verify docs generate correctly

[gemini-code-assist]

Code Review

This pull request adds a 'Gotchas' section to the documentation in lib/llm/index.js to address issues where multi-line SQL comments are misinterpreted as CLI flags. The review feedback correctly identifies that the sqlcmd query subcommand lacks a --query flag and suggests using the -- separator to properly handle positional arguments starting with dashes.