build: remove defunct test-postgres rule (#22839)

The `test-postgres` Makefile rule was redundant — CI never used it (it
runs `test-postgres-docker` + `make test` via the `test-go-pg` action),
and `make test` auto-starts a Postgres Docker container when needed via
`dbtestutil`.

- Remove the `test-postgres` rule from Makefile
- Update `pre-push` to run `test-postgres-docker` in the first phase
(alongside gen/fmt) and `make test` in the second phase
- Fix stale comments in CI workflows referencing `make test-postgres`
- Remove redundant "Test Postgres" entries from docs since `make test`
handles Postgres automatically

.claude/docs/WORKFLOWS.md

@@ -109,7 +109,6 @@
 
 - Run full test suite: `make test`
 - Run specific test: `make test RUN=TestFunctionName`
-- Run with Postgres: `make test-postgres`
 - Run with race detector: `make test-race`
 - Run end-to-end tests: `make test-e2e`