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 docs/ tree.

npkill is simple to use interactively but has real depth for scripted/automated use — profiles, a .npkillrc config file, a documented JSON/streaming output mode, and a whole separate Node.js API for embedding it in other tools (API.md). That's the part an agent is most likely to need help with (e.g. "write a script that deletes node_modules older than 30 days using npkill's JSON output"), and right now it's split across the README and three separate docs files with no single index pointing an agent at the right one.

Content is derived entirely from your existing docs (README sections: Installation, Usage, Options, Examples, JSON Output, Set Up Locally, Roadmap, plus docs/profiles.md, docs/npkillrc.md, docs/json-output.md, and API.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.