build: adopt Go 1.24 and use tool directives for dev tools

Migrate from the old tools.go pattern (with a separate build/ module)
to Go 1.24's native tool directive support.

Changes:
- Update go.mod to Go 1.24.0 with tool directive block
- Remove build/go.mod, build/tools.go, and related scripts
- Update Makefile to use 'go tool' instead of installed binaries
- GOCOVMERGE now uses '$(GO) tool gocovmerge'
- lint/format targets use '$(GO) tool golangci-lint/goimports'

Note: protoc plugins still require 'go install tool' because protoc
doesn't yet support 'go tool' directly.

Ref: https://go.dev/doc/modules/managing-dependencies#tools

.gitignore

@@ -32,3 +32,4 @@ _testmain.go
 *.exe
 *.test
 *.prof
+.venv/