fix(build): make .version writes resilient to missing git HEAD

[topitopongsala]

What

Wrap each git rev-parse HEAD > <dir>/dist/.version call in the build script with a subshell + fallback:

- && git rev-parse HEAD > browse/dist/.version
+ && (git rev-parse HEAD 2>/dev/null || echo unknown) > browse/dist/.version

(applied identically for design/dist/.version and make-pdf/dist/.version).

Why

When gstack is built outside its own git checkout — most notably Conductor's "Quick start → gstack template" flow, which clones gstack into a brand-new repo's .claude/skills/gstack/ (no .git of its own; parent repo has no commits, so HEAD is unborn) — git rev-parse HEAD exits 128 with:

fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.

The && chain then kills the entire build:

error: script "build" exited with code 128

…which surfaces in Conductor as Failed to create repo: error: script "build" exited with code 128 and the gstack template wipes the workspace before the user can inspect anything.

The .version files are pure metadata — they shouldn't be load-bearing on the build's success.

After this fix

• Normal checkout (your dev machine): still writes the real commit SHA. ✅
• Tarball / .git-less install / unborn parent HEAD: writes unknown and the build proceeds. ✅
• Conductor "Quick start → gstack template": completes successfully. ✅

Verification

# Normal case
cd /tmp && git clone https://github.com/garrytan/gstack && cd gstack
git checkout fix-build-script-git-rev-parse-resilience
bun install && bun run build
cat browse/dist/.version  # → real SHA, exit 0

# Conductor-template case (no .git)
tmp=$(mktemp -d) && cp -R . "$tmp"/ && rm -rf "$tmp/.git" "$tmp/node_modules"
cd "$tmp" && bun install && bun run build
cat browse/dist/.version  # → "unknown", exit 0 (previously: exit 128)

Out of scope

This PR doesn't touch the broader question of whether the install flow should hand gstack a real .git or copy .version files in some other way — it's the smallest possible patch to stop killing the build. Happy to follow up if maintainers prefer a different approach.