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/

.gitignore

@@ -1,5 +1,5 @@
 # build artifacts
-src/build/
+build/
 *.prg
 *.asm
 *.map