Move build output from src/build/ to ./build (repo root)

The build directory was under src/, which is unusual for a project
with a separate source tree. Moving it to the repo root's build/
directory follows the conventional project layout:

  ./build/           # build output (gitignored)
  ./src/             # source code only
  ./oscar64/         # compiler (submodule)

Changes:
- src/build.sh: BUILD_DIR changed from $SCRIPT_DIR/build to
  $ROOT/build
- .gitignore: replaced src/build/ with build/
- README.md: updated layout tree, build commands, and output path
- src/AGENT_CONTEXT.md: updated gitignore reference
- tasks.md: replaced all src/build/ with build/

src/AGENT_CONTEXT.md

@@ -155,7 +155,7 @@ $21-$24) goes to $D800-$DBE7.
 ## Commit conventions
 
 - One commit per phase, with a clear `Phase N: <summary>` prefix.
-- Don't commit generated build artifacts in `src/build/` (gitignored).
+- Don't commit generated build artifacts in `build/` (gitignored).
 - Generated data files (e.g. `src/data/processed/*.bin`) are checked
   in so the build doesn't depend on Python being installed.
 

src/build.sh

@@ -16,7 +16,7 @@
 #   ./build.sh -Os        # compile with -Os (optimize for size)
 #   ./build.sh -g         # compile with -g (adds source-level debug info)
 #
-# Output (in ./build/):
+# Output (in <repo>/build/):
 #   whack_hare.prg   — loadable C64 program (run with x64, VICE, or real hw)
 #   whack_hare.asm   — full 6502 listing
 #   whack_hare.map   — region/section/object placement
@@ -47,7 +47,7 @@ SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 ROOT="$(cd "$SCRIPT_DIR/.." && pwd)"
 OSCAR64_DIR="$ROOT/oscar64"
 OSCAR64_BIN="$OSCAR64_DIR/bin/oscar64"
-BUILD_DIR="$SCRIPT_DIR/build"
+BUILD_DIR="$ROOT/build"
 PRG="$BUILD_DIR/whack_hare.prg"
 D64="$BUILD_DIR/whack.d64"