docs: add CLAUDE.md documenting the local pre-push gate

CLAUDE.md

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+# CLAUDE.md
+
+Guidance for Claude Code sessions working in this repo.
+
+## What this is
+
+`splinter` (Supabase Postgres LINTER) maintains a set of SQL-based lints for
+Supabase Postgres projects. `bin/check_lints.py` checks every lint is fully
+registered, and `bin/compile.py` compiles all lint views into the tracked,
+generated `splinter.sql` (`UNION ALL` of every `lints/*.sql` view) - don't
+hand-edit `splinter.sql` directly, it's overwritten by `bin/compile.py`.
+
+## Build / test / lint
+
+```
+pre-commit run --all-files
+```
+
+Runs the full local gate, matching `.github/workflows/pre-commit_hooks.yaml`:
+`black` (pinned to `24.2.0` in `.pre-commit-config.yaml`, needs a `python3.12`
+interpreter on PATH for its hook env - pre-commit installs `black` itself, no
+manual install needed), `bin/compile.py` (regenerates `splinter.sql`),
+`bin/check_lints.py` (checks every lint is fully registered), plus standard
+hygiene hooks (trailing-whitespace, excluding `test/expected` since those
+`.out` files are whitespace-exact pg_regress golden files; check-added-large-files;
+check-yaml; mixed-line-ending; remove-tabs).
+
+```
+docker rmi -f dockerfiles-test
+SUPABASE_VERSION=15.1.1.13 docker-compose -f dockerfiles/docker-compose.yml run --rm test
+```
+
+Runs the real pg_regress test suite against a live Postgres, matching
+`.github/workflows/test.yml`'s effective behavior (which sets `PG_VERSION`,
+not `SUPABASE_VERSION` - a repo-side no-op both there and here, since
+`dockerfiles/docker-compose.yml` only reads `SUPABASE_VERSION` for its build
+arg). The `docker rmi -f` first is required, not optional: `docker compose
+run` reuses the last built image, so a lint/test/expected-file change without
+it can report a stale pass.