fix(cli): isolate concurrent credential temp files

[WNZhao]

Summary

• give every credential write a unique temporary path
• clean up the temporary file when an atomic write fails
• add a regression test covering 20 concurrent writes

Bug

writeCredentials previously reused credentials.json.tmp. Concurrent login/session writers raced on that file: one rename succeeded while the rest failed with ENOENT. In a local reproduction, 19 of 20 writes failed.

Verification

• pnpm exec vitest run packages/cli/src/credentials.test.ts (3 tests passed)
• concurrent reproduction after the patch: 20 fulfilled, 0 rejected

pnpm --filter @profullstack/sh1pt typecheck remains blocked by pre-existing unresolved workspace-package build outputs (for example @profullstack/sh1pt-core); the focused test passes.