In sandboxed macOS runtimes (e.g. Codex App), `security` / Keychain APIs
are blocked, so `keyring.Get`/`Set` for the DEK fails on every token
read/write. Add an opt-in env var that switches the macOS implementation
to the same file-based DEK scheme already used on Linux. Default
behavior is unchanged.
- Extract shared `fileDEK(service)` into `file_dek.go` (darwin || linux)
- Linux `getDEK` now delegates to `fileDEK`
- Darwin `getDEK` short-circuits to `fileDEK` when DWS_DISABLE_KEYCHAIN=1
- Document the tradeoff in reference.md (DEK and ciphertext co-located)
- Add darwin-only tests covering fallback path + overwrite
修雨