Fix cache path resolution to match prek's own behavior (#102)

## Summary

`restorePrekCache` used hardcoded platform paths (`~/.cache/prek` or
`LOCALAPPDATA/prek`) to save and restore the prek hook cache. A `prek
cache dir` CLI probe already existed in `prek.ts`, but it was only used
by `showVerboseLogs()` to locate `prek.log`. This change makes the CLI
probe the source of truth for cache save/restore as well, and fixes the
fallback to match prek's actual resolution order.

## Problems

- Cache path diverged from prek's actual cache location.
`getCachePaths()` in `cache.ts` hardcoded `~/.cache/prek` (Linux/macOS)
and `LOCALAPPDATA/prek` (Windows). prek itself resolves its cache via
`PREK_HOME`, then `XDG_CACHE_HOME`, then platform defaults. Any user
with `PREK_HOME` or a non-default `XDG_CACHE_HOME` would save/restore
against the wrong directory.
- CLI probe result was ignored for caching. `getPrekCacheDir()` already
queried `prek cache dir --no-log-file` (the `--no-log-file` flag
prevents the probe itself from writing to prek's trace log). But
`restorePrekCache` never called it, using the hardcoded paths instead.
- Duplicated, circular logic. `getCachePaths` lived in `cache.ts`,
`getPrekCacheDir` lived in `prek.ts`, and they imported each other for
their respective fallbacks.
- `os.arch()` vs `process.arch` inconsistency. The cache key used
`os.arch()` while the rest of the codebase uses `process.arch`. They
return the same value, but `process.arch` is the standard convention in
Node and avoids the `os` import.

## Fixes

- `restorePrekCache` now calls `getPrekCacheDir()`, which probes `prek
cache dir --no-log-file` first.
- Removes `getCachePaths()` entirely. The fallback logic is consolidated
into `getDefaultPrekCacheDir()` in `prek.ts`, mirroring prek's own
resolution: `PREK_HOME` (used as-is with tilde expansion, no `/prek`
suffix), then `LOCALAPPDATA/prek` on Windows, then
`XDG_CACHE_HOME/prek`, then `~/.cache/prek`.
- Replaces `os.arch()` with `process.arch` in the cache key.

## Future

The fallback exists because `prek cache dir` was added in v0.2.2 and
`--no-log-file` in v0.2.3, so the CLI probe fails on older versions
still in the version manifest (back to v0.0.23). Once pre-v0.2.2
versions are dropped from the manifest, `getDefaultPrekCacheDir` can be
removed entirely.

## Test plan

- [x] Unit tests for `getPrekCacheDir`: CLI probe happy path,
`PREK_HOME` fallback, `XDG_CACHE_HOME`/`LOCALAPPDATA` fallback

Author: shaanmajid