ci(postgres): flag division by a possibly-zero divisor in the compat guard (#56363)

Adds the division-by-zero divergence class to the PG-compat review tooling:
on a divisor that the data can drive to 0 (e.g. Sum(a)/Sum(b)), MariaDB
returns NULL for division by zero while PostgreSQL raises `division by zero`
and aborts the query. The portable fix is to wrap the divisor in
NullIf(divisor, 0), which yields NULL on both engines (matching MariaDB).

- .greptile/config.json: add it to the "would ERROR on PostgreSQL" list.
- .github/POSTGRES_COMPATIBILITY.md: document it under §1 (hard breaks).
- .github/helper/postgres_compat.py: note it in the docstring as a
  deliberately-not-statically-checked semantic divergence (data-dependent,
  like integer-division intent), so it stays a reviewer/Greptile concern.

Tooling-only; no source query changes. The instance fix shipped in #56361.

.github/POSTGRES_COMPATIBILITY.md

@@ -73,6 +73,11 @@ Flag a changed query that uses any of these:
   MariaDB's `IFNULL` is permissive. The common shape is `IfNull(date_col, 0) != 0 / == 0` as a presence test —
   replace with `date_col.isnotnull()` / `date_col.isnull()` (identical, and valid on both). Otherwise coalesce
   to a **same-type** default (`Coalesce(date_col, '1900-01-01')`, `Coalesce(text_col, '')`).
+- **Division by a possibly-zero divisor** — `Sum(a) / Sum(b)`, `x / col`, etc. where the
+  divisor can be `0`/empty. MariaDB returns `NULL` for division by zero; PostgreSQL raises
+  `division by zero` and aborts the query. Wrap the divisor in `NullIf(divisor, 0)` — that
+  yields `NULL` on both engines, matching MariaDB's value. (Only the *literal* `/ 0` is a parse
+  constant; the trap is a divisor that is an aggregate or column the data can drive to zero.)
 
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