feat(docs): 907 — host an llms.txt for coding agents installing jarl

[randomdevpete]

Summary

Add packages/docs/public/llms.txt, hosted at https://jarl.randomdev.co.uk/llms.txt, to guide coding agents through installing and using the library.

Convention Research

The producer verified the file format and availability by:

• Researching the current convention for hosted, consumer-facing agent instructions: llms.txt is now the standard format, adopted by OpenAI, Stripe, Cloudflare, Vercel, and the most comparable open-source libraries (TanStack and Zod).
• Fetching live examples from those projects to confirm the format: H1 heading with library name, blockquote summary, then ## section headings with [Name](url): description links to existing documentation sections — not a full-text dump.
• Examining and rejecting AGENTS.md as the vehicle: that convention is for agents working inside a repo as contributors (which this repo covers via CLAUDE.md), not for downstream consumers reading from their own codebase with no jarl source.
• Examining and rejecting an MCP server: disproportionate scope for a routing library that already has a docs site. The routing behaviour is documented by code and examples, not through a separate tool interface.
• Noting but deferring llms-full.txt: it would require a build-time markdown concatenation step, materially beyond the scope of "write and host some static instructions". Possible as a follow-up if agents prove they need it.

The llms.txt format is deterministic and content-driven: a single static file served verbatim at a standard path, requiring no build step and no API layer, making it visible to any tool that can fetch a URL.

Verification

• All documentation links in the file have been verified against the docs site's actual route table.
• Every factual claim about package names, versions, exports, and peer dependencies has been verified against the current source.
• No unlanded tickets (675, 778, 789) are documented — their proposed API changes (async-data coalescing, navigation blocking, fooAtom/fooRouteAtom naming) are absent.
• CODING-STYLE.md compliance: no ticket/epic/PR references, no padding, no marketing fluff.

See ticket 907 for the done-when criteria.