feat(moa): default advisor fanout to user_turn — the cheapest cadence

Flips the default fan-out cadence from per_iteration (advisors re-run on
every tool iteration, multiplying advisor spend by tool-loop depth) to
user_turn (advisors run once on the first message of each user turn; the
acting aggregator works the rest of the tool loop with that turn's
advice). Until per-mode benchmarks justify a costlier default, MoA
defaults to the cheapest, lowest-impact cadence (#67199).

One default for everyone — no split legacy/new-preset semantics; presets
that want per-step advising set fanout: per_iteration explicitly. All
three modes (user_turn / per_iteration / every_n:N) remain selectable;
every_n:1 still collapses to per_iteration (semantic identity), while
unparseable values now fall to user_turn (the default).

Docs updated with a default-change note; the per-iteration rerun test
pins its mode explicitly.

Co-authored-by: skyer-flyyy <188930297+skyer-flyyy@users.noreply.github.com>

web/src/lib/api.ts

@@ -2349,7 +2349,7 @@ export interface MoaConfigResponse {
     max_tokens: number;
     /** Optional advisor output cap — round-tripped, not edited here. */
     reference_max_tokens?: number | null;
-    /** Fan-out cadence (per_iteration | user_turn) — round-tripped. */
+    /** Fan-out cadence (user_turn default | per_iteration | every_n:N) — round-tripped. */
     fanout?: string;
     enabled: boolean;
   }>;