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.

README.md

@@ -21,19 +21,19 @@ original program structure and the design of this port.
 Requires Go 1.25 or later and a terminal at least 80x24.
 
 ```bash
-go build ./cmd/rogue
-./rogue
+make build
+./build/rogue
 ```
 
 ```bash
 # Restore a saved game
-./rogue ~/rogue.save
+./build/rogue ~/rogue.save
 
 # View high scores
-./rogue -s
+./build/rogue -s
 
 # Test the death screen (demo mode)
-./rogue -d
+./build/rogue -d
 ```
 
 ## In-game commands
@@ -57,7 +57,7 @@ Press `?` in game for the full list.
 export ROGUEOPTS="name=YourName,terse,jump,fruit=mango"
 
 # Wizard (debug) mode, with a reproducible dungeon
-ROGUE_WIZARD=1 SEED=12345 ./rogue
+ROGUE_WIZARD=1 SEED=12345 ./build/rogue
 ```
 
 The scoreboard is kept in `~/.rogue.scores`. Save files are Go gob snapshots
@@ -80,9 +80,9 @@ For development, the `Makefile` wraps the toolchain: `make fmt` (gofmt +
 prettier), `make lint` (`script/lint`, which runs golangci-lint inside the
 pinned container built from `Dockerfile.lint` — it is never installed on the
 host, so docker is required), `make test` (the suite, under the race detector
-with coverage and a timeout), and `make check` (all three). Use the targets
-rather than invoking `go test` directly — they carry the flags the project
-relies on.
+with coverage and a timeout), `make check` (all three), and `make build` (the
+executable, into the git-ignored `build/`). Use the targets rather than the
+toolchain directly — they carry the flags the project relies on.
 
 ## License