docs: document fullText contains query in drive_search tool description

[aliwatters]

Summary

The drive_search tool already supports Google Drive's fullText contains query operator (it passes the query directly to the API), but the tool description doesn't mention it. This means AI agents don't know they can search file contents.

Current State

The tool description in internal/drive/register.go only shows examples like name contains 'report' and mimeType = 'application/pdf'. Users/agents don't realize they can search inside file contents.

Proposed Change

Update the drive_search tool description and query parameter description to mention fullText contains:

• Tool description: mention content search capability (PDFs, Docs, Office files, etc.)
• Query parameter: add fullText contains 'keyword' as an example

What fullText contains Searches

• File content (text inside PDFs, Docs, Sheets, Slides, Office formats, HTML, plain text)
• File name, description, and indexableText metadata
• Google auto-indexes content including OCR for scanned PDFs

Caveats to Document

• Whole-token matching only ('sta' won't match 'stan')
• Known pagination duplication bug (https://issuetracker.google.com/issues/391672293) — consider deduplicating results by file ID
• Indexing delay for newly uploaded files

References

• Drive API search query terms
• Search for files and folders

[aliwatters]

Fixed in d577f39 — drive_search description now documents fullText contains for content search.