macOS: 默认的 `projects_dir` 位于 `~/Documents` 中, iCloud 会同步此目录

**Detect and avoid.** On macOS, `get_default_projects_dir()` unconditionally resolves to `~/Documents/PlatformIO/Projects`. When a user has iCloud Drive's "Desktop & Documents Folders" enabled — an option macOS actively prompts for during setup — that directory is managed by `fileproviderd`. PlatformIO then writes `.pio/` build trees, hundreds of MB of churn per rebuild, into a live sync target. This produces build and toolchain failures that are hard to attribute, because the sync daemon acts *after* the operation reports success.