build: add a make build target, drop README's raw go build (closes #19)

The executable builds to build/rogue. All generated artifacts go under
build/, which .gitignore covers as a whole; a target writing outside it
can commit its output.

build is in neither check nor test: check stays fmt-check lint test and
still writes nothing into the working tree.

README's "Building and running" block and the run examples use
./build/rogue, and no raw go invocation is left in the file.

Verified: make build writes build/rogue and git status stays clean;
GOFLAGS=-count=1 make check green in 34s with the lint layer executing
(21.9s, "0 issues.", not CACHED) and the suite running for real (cmd/rogue
1.027s, game 3.450s), git status clean afterwards.

Makefile

@@ -6,15 +6,27 @@
 # around the Go toolchain and prettier so `make fmt` / `make check` behave
 # the same as in sneak's other repos.
 
-GO_PKGS  := ./...
-MD_FILES := $(shell git ls-files '*.md')
-PRETTIER := prettier --tab-width 4 --prose-wrap always
+GO_PKGS   := ./...
+MD_FILES  := $(shell git ls-files '*.md')
+PRETTIER  := prettier --tab-width 4 --prose-wrap always
 
-.PHONY: check fmt fmt-check lint test
+# Every generated artifact goes here, and the whole directory is
+# git-ignored. Targets that write outside it can commit their output.
+BUILD_DIR := build
+BIN       := $(BUILD_DIR)/rogue
 
-# Format, lint, and test — the full local pre-commit gate.
+.PHONY: build check fmt fmt-check lint test
+
+# Format, lint, and test — the full local pre-commit gate. Keep this list
+# to targets that write nothing into the working tree.
 check: fmt-check lint test
 
+# Build the executable into $(BUILD_DIR). `go build -o` does not create the
+# parent directory.
+build:
+	@mkdir -p $(BUILD_DIR)
+	go build -o $(BIN) ./cmd/rogue
+
 # Format Go and Markdown in place.
 fmt:
 	gofmt -w .