Add CLAUDE.md, AGENTS.md, and INDEX.md — all generated by the indxr
MCP tooling to give AI coding assistants a structured way to explore
the codebase without dumping entire files into context.
CLAUDE.md is the detailed version with token cost estimates and a
full tool reference. AGENTS.md is the condensed version. INDEX.md
is an auto-generated codebase index with file summaries and symbol
maps.
While at it, add .indxr-cache/ to .gitignore because nobody needs
that in the repo.
