Add AI-accessible tools for reading structured files (CSV, JSON/JSONC,
Markdown, SQL) and searching repository content (git_grep). Also adds
git_blob_get to retrieve raw blob text content with binary detection.
Deferred: Excel, Word, PDF, PPT modules are stubbed out due to library
API incompatibilities (calamine 0.26, lopdf 0.34, quick-xml 0.37).
