AIGX 1.3.1 — editor ecosystem (.aigx highlights everywhere)

.aigx now highlights everywhere — including on GitHub itself — all from one canonical grammar.
Spec stays 1.1; this is an editor-ecosystem release.

Highlighting on GitHub

• .gitattributes renders .aigx with XML highlighting on GitHub today (kept out of the language bar).
  Copy the one line into any repo that adopts AIGX.
• editors/linguist/ — a complete kit to
  register AIGX as a first-class Linguist language (languages.yml entry, sample, step-by-step PR guide,
  tm_scope: source.aigx).

More editors (one canonical grammar, many targets)

• Sublime Text — native aigx.sublime-syntax.
• TextMate — AIGX.tmbundle wrapping the grammar.
• Zed — extension +
  tree-sitter grammar source
  (experimental — needs tree-sitter generate).
• editors/README.md — the ecosystem index.

(VS Code / Cursor / Windsurf already have the full extension from 1.3.0.)

Notes

• No package versions changed. The meta-genome rule ARCH-grammar-canonical now keeps every derived grammar
  (VS Code copy, tmbundle, Sublime, tree-sitter) in sync with the canonical TextMate grammar.
• All three reference validators (Python · Node · Rust) still agree on the conformance suite.