✨ Add buf + goose, fix PATH order, rewrite README ✨

Add buf (modern protobuf toolchain) and goose (Go-native DB migration
runner) to complete the Go dev toolchain. Fix PATH order so baked image
tools in /usr/local/bin always take precedence over anything installed
at runtime into $GOPATH/bin. Full README rewrite following canonical
section order.
- Dockerfile: insert /usr/local/bin between /usr/local/go/bin and
/usr/local/share/go/bin in both build and final stage ENV PATH
- rootfs/etc/profile.d/go.sh: same PATH fix; explicit /usr/local/bin
guard added; $GOPATH/bin appended last
- rootfs/root/docker/setup/05-custom.sh: add buf@latest and
goose/v3@latest installs with descriptive comments
- README.md: full rewrite — H1 title, canonical section order (Pull,
Docker, Tools table, Env vars, PATH order, Persistence, Cross-compile,
Development, License); accurate tool list matching actual image content

Dockerfile
README.md
rootfs/etc/profile.d/go.sh
rootfs/root/docker/setup/05-custom.sh

rootfs/root/docker/setup/05-custom.sh

@@ -132,6 +132,12 @@ go install google.golang.org/protobuf/cmd/protoc-gen-go@latest
 # gRPC Go code generator
 go install google.golang.org/grpc/cmd/protoc-gen-go-grpc@latest
 
+# Modern protobuf toolchain: lint, format, breaking-change detection, managed deps
+go install github.com/bufbuild/buf/cmd/buf@latest
+
+# DB schema migration runner (Go-native, supports Go and SQL migrations)
+go install github.com/pressly/goose/v3/cmd/goose@latest
+
 echo "Go tooling installed"
 
 # Strip the module download cache and ephemeral build cache from this layer.