Why

Auto Review should remain the effective approval reviewer when settings cross runtime boundaries. A config or app-server round trip must not change the reviewer identity, and delegated work must not silently fall back to user review.

This requires both a stable canonical serialized value and propagation of the effective setting. auto_review is the canonical value across protocol and app-server output, while guardian_subagent remains accepted as backward-compatible input.

What changed

• serialize ApprovalsReviewer::AutoReview consistently as auto_review across core protocol and app-server v2
• continue accepting guardian_subagent when reading existing config or client requests
• carry the active turn's approval reviewer into spawned agents
• update config/debug expectations and add delegated-task regression coverage

Scope

This does not change Guardian policy or remove compatibility with existing guardian_subagent inputs. It preserves the selected reviewer across serialization, config reloads, app-server settings, and delegated task setup.

Related Guardian changes are split independently:

• feat: distinguish Guardian denials and soft denials #26231 adds denials and soft denials
• fix: retry transient Guardian reviewer failures #26334 retries transient reviewer failures
• feat: reuse low-risk Guardian approvals #26333 reuses narrowly scoped low-risk approvals
• feat: add Guardian denial recovery TUI #26232 adds TUI denial recovery

Validation

• just test -p codex-app-server-protocol (224 passed)
• regression coverage for delegated task reviewer propagation
• serialization coverage for canonical auto_review output and legacy guardian_subagent input