Add llms.txt for LLM/agent-readable project summary

[abyworkings-coder]

This repo doesn't have an llms.txt file yet — a small, curated markdown index (title, one-line summary, and grouped links to the docs that matter) that AI coding agents can read to understand a project quickly, instead of having to crawl and guess through the full README and doc/ tree.

fq has a genuinely large surface area — a custom jq-derived query language, 100+ format decoders, and CLI flags that overlap with but aren't identical to jq's — which is exactly the kind of project where an agent benefits from a short index pointing straight at doc/usage.md and doc/formats.md instead of inferring behavior from README examples alone. Given fq's own pitch is "jq for binary formats," its users are already comfortable with the idea of tools built for programmatic/scripted use, which made this feel like a natural fit.

Content is derived entirely from your existing docs (README sections on install, background, goals, and supported formats, plus doc/usage.md and doc/dev.md) — no new claims, just pointers. Validated against the llms.txt spec with llms-txt-lint (disclosure: a small validator tool I maintain — happy to drop that line from the PR if you'd rather not have it).

Totally understand if this isn't something you want to maintain — no worries either way, feel free to close.