Add the prose rules to AGENTS.md

[pbrubeck]

Description

AI-assisted (Claude Code)

AGENTS.md only, no code. Companion to firedrakeproject/firedrake#5338, which makes the same two
additions on the Firedrake side.

FIAT's Style and Conventions section already matches Firedrake's on class attributes, the hasattr
prohibition, type hints and numpydoc. It carries neither prose rule, so agent-written prose here is
unconstrained.

Main changes

• Plain English (ASD-STE100) for docstrings and comments, with a Clause-Stacked Docstrings And
  Comments anti-pattern. Unedited AI prose reliably produces garden-path sentences that bury the
  subject in a relative clause.

• Documenting Code That Is Not There. The case worth naming is not "this used to do X", which
  nobody writes, but an argument against a branch that was just removed:

  # A test for a repeated point here would divide by zero.

  That reads as present tense while describing absent code. The rule carries a test a reader can
  apply — a reader who never saw the diff must not be able to tell that anything was removed — and
  the words that give it away, including would when its subject does not exist.

This adds the first ## Anti-Patterns section to FIAT's AGENTS.md. Both examples use small FIAT-
flavoured functions rather than real code, so they do not go stale.

Why examples rather than prose

The rules with WRONG/RIGHT pairs get followed; the ones stated only as prose get missed. On the
Firedrake side, Document The Present, Not The Past was violated during the work that produced
firedrakeproject/firedrake#5337 while the rule was sitting in the file, and a human caught it rather
than the rule. These two are also mechanically checkable, and I am running a local PostToolUse hook
that greps added lines for the Sphinx field-list style, over-long sentences, the past-tense tells,
and the hasattr guard.