docs: note markdown code fences are type-checked

[tomas-zijdemans]

The author of #7289 validated their README change with deno fmt --check and CI still failed: deno task test runs with --doc, which type-checks ts fences in markdown files, and no doc mentions that. This adds one line to AGENTS.md so the next drafter knows to mark non-runnable snippets ```ts ignore.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.03%. Comparing base (ca58f94) to head (42ef215).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7291   +/-   ##
=======================================
  Coverage   95.03%   95.03%           
=======================================
  Files         617      617           
  Lines       51637    51637           
  Branches     9359     9359           
=======================================
  Hits        49075    49075           
  Misses       2021     2021           
  Partials      541      541           

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