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.

TODO.md

@@ -35,6 +35,16 @@ is finished.
 
 # Completed Steps
 
+- 2026-08-10 `make build` added (https://git.eeqj.de/sneak/rgoue/issues/19). The
+  executable is built to `build/rogue`; `README.md` no longer contains a raw
+  `go` invocation anywhere. `build` is in neither `check` nor `test` —
+  `make check` stays `fmt-check lint test` and still writes nothing into the
+  working tree.
+
+    Generated artifacts now all live under `build/`, which `.gitignore` covers
+    as a whole. Anything written outside it is committable, so a target that
+    puts its output elsewhere reintroduces the stray-artifact problem.
+
 - 2026-08-10 Linting moved into a container
   (https://git.eeqj.de/sneak/rgoue/issues/41). `golangci-lint` is no longer
   invoked on the host anywhere in the repo: `Dockerfile.lint` pins