ci(postgres): add a static pre-commit check for MySQL-only SQL

The Postgres test job is label-gated, so it does not run on every PR. This adds an
always-on pre-commit hook that statically flags the *mechanical* breaks: MySQL-only
functions (timestamp(date,time), timediff, str_to_date, date_format/add/sub,
group_concat, period_diff, SQL IF()), SHOW INDEX/TABLES/COLUMNS, single-quoted
aliases, UPDATE..JOIN, interpolated/f-string SQL carrying MySQL-isms,
set_value/db_set(<Check>, bool), and MySQL SHOW INDEX result keys.

It deliberately does NOT flag the framework auto-translations (ifnull->coalesce,
backtick/locate/REGEXP, .like()->ILIKE) nor the *semantic* divergences (loose GROUP
BY, case-sensitive ==/IN, NULL ordering, tiebreakers) — those need the test suite,
which remains the backstop. AST + structure-gated regex keep false positives near
zero (docstrings and prose skipped); '# pg-ok' exempts intentional MariaDB-only
branches. Scoped to erpnext/ excluding patches/. Includes a unit test of the checker.

.pre-commit-config.yaml

@@ -66,6 +66,18 @@ repos:
       - id: ruff-format
         name: "Run ruff formatter"
 
+  - repo: local
+    hooks:
+      - id: postgres-compat
+        name: "PostgreSQL compatibility (static check)"
+        description: "Flags MySQL-only SQL that breaks on Postgres; the label-gated PG test job is the backstop for semantic divergences."
+        entry: .github/helper/postgres_compat.py
+        language: script
+        files: ^erpnext/.*\.py$
+        # patches/ are historical, version-gated migrations (skipped on fresh Postgres installs);
+        # out of scope for the always-on gate. test_postgres_compat.py holds intentional bad-SQL fixtures.
+        exclude: ^erpnext/patches/|^erpnext/tests/test_postgres_compat\.py$
+
 ci:
     autoupdate_schedule: weekly
     skip: []