Docs: add llms.txt and fix RegisterContract arity in smart contract guide

[AndreaRaccagni]

What

1. Adds llms.txt and llms-full.txt at the docs site root, per the
   llmstxt.org spec, so AI coding assistants (Claude, ChatGPT,
   Cursor) can ground answers about Magi in accurate context.
2. Fixes the RegisterContract example in the smart-contract
   development guide to use the correct 3-arg signature
   (id, owner, wasm). Reference page already had the correct
   version; the how-to guide was out of date.

Files

• public/llms.txt (new) — slim curated index per llms.txt spec
• public/llms-full.txt (new) — expanded version with SDK + how-to inlined
• src/content/docs/How To/smart contract development.mdx — one-line fix

[vercel]

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