gitignore: exclude the entire build directory

Replace the per-subdir entries with a single /build/ to cover any
new outputs Make or scripts add later (build/manual exists today;
future docs/coverage variants would otherwise need new lines).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -1,7 +1,5 @@
 state/
-/build/bin/
-/build/smoke/
-build/manual/
+/build/
 /runtime/
 /dist/
 /banger
@@ -21,8 +19,4 @@ id_rsa
 /todos
 /coverage.out
 /coverage.html
-/build/unit/
-/build/combined/
-/build/combined.cover.out
-/build/combined.cover.html
 /.codex