Test race: platform-binary.test.ts creates src/node_modules while api-deprecated-paths.test.ts scans src/ #694

What happens

A full local `pnpm test` can fail intermittently in `packages/cli/test/api-deprecated-paths.test.ts` with `ENOENT` on `packages/cli/src/node_modules`. The test passes when run alone, and CI hasn't hit it yet.

Why

• `platform-binary.test.ts` ("reports node_modules/.bin as the source when that tier answers", around line 208–221) creates `packages/cli/src/node_modules` as a fake workspace, then removes it.
• `api-deprecated-paths.test.ts` walks `SOURCE_ROOT` (`packages/cli/src`) recursively with `readdir` (around line 32/52).
• Vitest runs the files in parallel. When the walk lists `src/node_modules` and then descends into it after the other test has removed it, the walk throws `ENOENT`.

Found while iterating on #689; it isn't caused by that PR and is already on `main`.

Done when

• `platform-binary.test.ts` no longer writes inside `packages/cli/src`. It should use a temp directory, or a fixture location the source walk never visits.
• The source walk in `api-deprecated-paths.test.ts` skips `node_modules` (and tolerates entries that vanish mid-walk), so a stray directory can't break it.
• Running both files together repeatedly (e.g. `pnpm exec vitest run test/platform-binary.test.ts test/api-deprecated-paths.test.ts` in a loop) passes every time.