Failed mkc/npm commands hide their output, even with --debug #15

When a command run through run_cmd fails, its stdout and stderr are stored in BuildError.output, but the exception message is only <command> failed (<code>) in <cwd>. Nothing logs .output, and neither redun's traceback nor main() prints it. Successful output isn't logged either, even with --debug.

Relevant code: utils/run.py, lines 12–22 and lines 52–56.

As a result, a game's compile errors and any npm ci/npm run build failure can't be diagnosed from the tool's output.

Reproduction in an isolated MKCD2APP_HOME:

1. Install a fake mkc in the toolchain's js_tools/node_modules/.bin that prints main.ts(3,1): error TS2304: Cannot find name 'spritez'. UNIQUE_MARKER_123 to stderr and exits 1, and put a placeholder ---simulator.html in the target dir.
2. Run mkcd2app build on a path source config, then run it again with --debug.
3. Both exit 1 with BuildError: ['mkc', 'build', '-j'] failed (1) in .../g-binary-js-build. UNIQUE_MARKER_123 appears in neither stdout nor stderr.

Real-world case: with the real toolchain, mkcd2app target install 4.0.999 fails with only BuildError: ['mkc', 'build', '-j'] failed (1) in /tmp/tmp8a_mvs_t. That temporary directory has already been deleted, so there's nothing left to inspect.

The tests check exc.value.output (test_build_wiring.py, lines 199–201) but not what the user sees.

Related gaps in the same function: output isn't streamed, so a long npm ci shows no progress, and there's no timeout.

Expected behavior: a failed command's output (or at least its tail) is shown to the user, in the exception message or logged at error level, and the full output is available with --debug.

Found while reviewing PR #7, toolchain at ce12c48.