Remove erpnext/tests/test_postgres_compat.py (and its pre-commit exclude); a unit
test for the dev-tooling lint helper isn't needed in the app test suite.
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.
