Fix gstack build when Conductor creates a repo with unborn HEAD

[Cloverings1]

This PR fixes a Conductor Quick Start failure where the gstack template build exits with code 128 before the generated repo has an initial commit.

Root cause: the build script calls git rev-parse HEAD while the repo can still have an unborn HEAD. That makes setup fail even though the upstream gstack repo itself has a valid HEAD.

Changes:

Adds scripts/write-build-version.sh
Falls back to unknown when HEAD does not resolve
Writes the resolved version to all .version files
Adds regression coverage for both unborn HEAD and normal committed repos

[Cloverings1]

Additional verification after reproducing this locally through Conductor:

• Conductor Quick Start fails because the generated repo can be in an unborn-HEAD state before the first commit exists.
• A local workaround that only makes git rev-parse HEAD return unknown for unborn repos allowed Conductor's GStack Quick Start to complete.
• This PR fixes the same root cause inside GStack itself, without requiring a local Git shim.

I also tested the PR rebased onto current origin/main without the local workaround in PATH:

• bun test test/write-build-version.test.ts test/setup-codesign.test.ts passes: 8 pass, 0 fail.
• Full bun run build passes inside a fresh Git repo with no commits.
• The generated version files contain unknown in that unborn-HEAD case.

So the fix still preserves normal commit SHA behavior while allowing Conductor/new-repo bootstrap flows to finish before the first commit exists.

[Cloverings1]

Updated this PR against current main.

It now keeps the current vendor:xterm build step and moves the version-file writes behind scripts/write-build-version.sh, which falls back to unknown when git rev-parse --verify HEAD cannot resolve.

I also added regression coverage for both observed bootstrap failure modes:

• a Git repo before the first commit exists / unborn HEAD
• a checkout where Git metadata is not present, matching the Conductor trace where .git is removed before setup

Focused validation passes locally:

bun test test/write-build-version.test.ts test/setup-codesign.test.ts
# 9 pass, 0 fail

I tried a full bun run build as well, but this fresh clone did not have dependencies installed yet, so it stopped before the relevant version-writing step because node_modules/xterm / marked were missing. The focused regression path above is passing.