vp migrate hangs indefinitely (0% CPU) when the project's pinned pnpm deadlocks during install

## Summary

`vp migrate` can hang **indefinitely at 0% CPU** (no output, no progress, no timeout) on some projects. The hang is in the dependency install that migrate spawns: the project's **pinned pnpm 9.x** deadlocks on an internal worker pipe. Because `vp migrate` has no timeout or hang detection, the command never returns and must be killed manually.

Discovered while smoke-testing the v0.2.2 prerelease (PR #2016, registry-bridge commit `1d7ba811f0e456640b0e761e8e8acf77d1ad3251`) across ~64 real-world projects: 2 of them hang every time, the other ~62 migrate fine.

## Affected projects (100% reproducible)

| Project | Pinned pnpm | Node |
| --- | --- | --- |
| `anywidget` | `pnpm@9.10.0` | 24.18.0 |
| `vitepress-carbon` | `pnpm@9.9.0` | 22.18.0 (`.node-version`) |

Both are pnpm monorepos. ~62 other projects in the same run (mostly newer pnpm) migrated successfully through the same registry bridge, so this is specific to these projects' resolution + their old pinned pnpm.

## Symptom

`vp migrate` prints nothing after startup and never returns:

```
Running vp migrate in /Users/.../anywidget
<-- hangs here forever; killed after 20 min every time -->
```

- The `vp` process and its `node`/`pnpm` children all sit at **0.0% CPU** (idle deadlock, not a busy loop).
- Not a stdin prompt: reproduces identically with `stdin` redirected from `/dev/null`.
- Not concurrency: reproduces when run solo with nothing else installing.

## Root-cause evidence

The hung process is the install that migrate spawns:

```
node .../pkg-pr-new//package_manager/pnpm/9.10.0/pnpm/bin/pnpm.cjs install --no-frozen-lockfile
```

**Stack sample of the hung node/pnpm main thread** (`sample `):

```
node::SpinEventLoopInternal
uv_run
uv__io_poll
kevent <-- main thread parked in the event loop waiting on an fd
```

**`lsof` of the hung process** shows **no TCP/network connections — only internal PIPE fds**:

```
node ... 4 PIPE 0x...->0x...
node ... 5 PIPE 0x...->0x...
node ... 12 PIPE 0x...->0x...
node ... 13 PIPE 0x...->0x...
```

So it is **not** stalled on the registry/network. pnpm's main process is blocked in the event loop waiting on an internal worker pipe that never signals, i.e. an internal pnpm worker-IPC deadlock (same class as the pnpm 11.0.6 "hangs after Done" bug that was fixed in 11.5.1, but here on pnpm 9.x).

## Reproduction

```bash
# via the pkg.pr.new bridge migrate helper
ALLOW_DIRTY=1 .github/scripts/test-pkg-pr-new-migrate.sh \
1d7ba811f0e456640b0e761e8e8acf77d1ad3251 /path/to/anywidget --no-interactive
# ...or plainly: cd anywidget && vp migrate
# hangs at "Running vp migrate ..." with 0% CPU; sample/lsof the pnpm child as above.
```

## Impact

- `vp migrate` never completing is a poor failure mode: no error, no timeout, silent 0%-CPU hang; a CI or a user waits forever.
- In an automated fleet run it also leaves **orphaned hung pnpm processes** behind (observed several, up to 1h+ old) that must be reaped manually.

## Suggested directions

1. **Add a timeout / hang detection to the install step of `vp migrate`** so it fails loudly (with the pnpm command + partial output) instead of hanging silently forever. This alone converts the worst part (silent infinite hang) into an actionable error.
2. Consider running the migrate install with **vp's managed/bundled pnpm** (newer) rather than the project's old pinned `pnpm@9.x` that deadlocks, or expose a flag to opt into that.
3. Investigate the underlying pnpm 9.x worker-pipe deadlock on these two projects' graphs (likely reproducible with a plain `pnpm@9.10.0 install` on `anywidget`).

_Found during the v0.2.2 real-world test; see https://github.com/voidzero-dev/vite-plus/pull/2016 for the prerelease build under test._