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/docs tree.

Given ccstatusline's audience is largely Claude Code / agent users configuring their own tooling, this felt like a natural fit — an agent asked to help someone customize their statusline could load this file directly instead of parsing the full (quite long) README changelog.

Content is derived entirely from your existing docs (README Quick Start/Contributing sections, docs/WINDOWS.md, docs/USAGE.md, docs/DEVELOPMENT.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.