bun build --compile fails on virtiofs mounts (devcontainers)

## Summary

`bun build --compile` fails inside devcontainers that use **virtiofs**-mounted filesystems. This breaks the `./setup` script — the browse binary at `browse/dist/browse` is never rebuilt, leaving a stale binary that can't locate `server.ts`. The result is every `$B` command hanging with:

```
[browse] Another instance is starting the server, waiting...
[browse] Timed out waiting for another instance to start the server
```

## Root Cause

Bun's `--compile` flag writes output to a temp file (e.g. `/run/host_virtiofs/.../.bun-build`) and then does an atomic `rename()` to the target path. On virtiofs mounts, this rename fails with `ENOENT`:

```
[9ms] bundle 2 modules
failed to rename /run/host_virtiofs/.../.189f99fd7dcbfffb-00000000.bun-build to browse/dist/browse: ENOENT
```

The `setup` script has `|| true` at the end of the build pipeline, so this failure is silently swallowed and setup reports success:

```
gstack ready (claude).
browse: /path/to/browse/dist/browse
```

The stale compiled binary has wrong embedded `import.meta.dir` and `process.execPath` values, so `resolveServerScript()` in `cli.ts` can't find `server.ts`. The server process never starts, and `acquireServerLock()` in `ensureServer()` times out waiting for a state file that will never appear.

## Reproduction

1. Run VS Code / Cursor with a devcontainer on macOS (Docker Desktop with virtiofs)
2. Install gstack and run `./setup`
3. Observe `bun build --compile` failure in output (easy to miss due to `|| true`)
4. Run any `$B` command → hangs and times out

## Workaround

Replace the compiled binary with a shell wrapper that delegates to `bun run`:

```sh
#!/bin/sh
SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
exec bun run "$SCRIPT_DIR/../src/cli.ts" "$@"
```

This works because `bun` itself runs fine on virtiofs — only `bun build --compile` is broken (it's a known Bun issue with cross-mount atomic renames).

## Suggested Fix Options

1. **Shell wrapper fallback in `setup`**: After `bun build --compile` fails, detect the failure and write the shell wrapper automatically. This is the simplest fix and works everywhere.

2. **Build in `/tmp` and copy**: `cd /tmp && bun build --compile .../cli.ts --outfile /tmp/browse && cp /tmp/browse .../browse/dist/browse` — avoids the virtiofs rename issue since `/tmp` is a normal filesystem.

3. **Detect virtiofs and skip compile**: Check if the working directory is on a virtiofs mount and use the wrapper approach by default.

Option 1 is probably the most robust since it handles any filesystem where `bun build --compile` fails, not just virtiofs.

## Environment

- Docker Desktop with virtiofs (macOS host → Linux container)
- Bun 1.3.x
- The issue is upstream in Bun ([oven-sh/bun#4253](https://github.com/oven-sh/bun/issues/4253)) but the gstack setup should handle it gracefully