Fix Windows build artifact finalization

[McGluut]

Summary

This PR moves the build artifact finalization out of shell subshells and into a small Bun script.

Changes:

• write the .version files from scripts/finalize-build-artifacts.ts
• mark built executables from that script when they exist
• remove root .bun-build temp artifacts without package-script shell syntax
• make the build stop if xterm vendoring or skill-doc generation fails
• add a focused test for the finalizer

This should address #1537 and #1538.

Validation

• bun install --frozen-lockfile
• bun run build
• bun test test/finalize-build-artifacts.test.ts
• git diff --check

[time-attack]

@16francej Please close this PR. Windows build finalization, version files, permissions, cleanup, and failure handling are all owned by the newer #1594 build pipeline. This older implementation is fully superseded.

[capy-ai]

Thanks for this, @McGluut. The fix is now on main: the build moved to scripts/build.sh (bash, set -e), so the Bun-shell subshell redirects are gone (already on main as of v1.91.16.0). Closing as superseded; if you still see the problem on the latest gstack, please open an issue with ./setup --status output.