Remove lint.sh; re-add 'make fmt'

See Makefile for explanation on difference between 'make fmt' and 'make lint'.

I also removed lint.sh. Original motivation was to use it in aider, but it's not
a good fit there, because aider passes filenames and it does not work well with
golangci-lint which requires whole packages to work.

Follow up to #2062.

Makefile

@@ -5,11 +5,18 @@ PACKAGES=./acceptance/... ./libs/... ./internal/... ./cmd/... ./bundle/... .
 GOTESTSUM_FORMAT ?= pkgname-and-test-fails
 
 lint:
-	./lint.sh ./...
+	golangci-lint run --fix
 
 lintcheck:
 	golangci-lint run ./...
 
+fmt:
+	# Note 'make lint' will do formatting as well. However, if there are compilation
+	# errors, formatting/goimports will not be applied by 'make lint'. However, it will be
+	# applied by 'make fmt'.
+	# If you need to ensure that formatting & imports are always fixed, do "make fmt lint"
+	golangci-lint run --enable-only="gofmt,gofumpt,goimports" --fix ./...
+
 test:
 	gotestsum --format ${GOTESTSUM_FORMAT} --no-summary=skipped -- ${PACKAGES}
 
@@ -39,4 +46,4 @@ integration:
 integration-short:
 	$(INTEGRATION) -short
 
-.PHONY: lint lintcheck test cover showcover build snapshot vendor schema integration integration-short
+.PHONY: lint lintcheck fmt test cover showcover build snapshot vendor schema integration integration-short