Apply an analysis recommendation to the gate config

Use when: caller wants to accept a specific recommendation from a session’s analysis (e.g. tweak temperature, swap a model) and have Layer apply it to the gate’s config

Applies an analyzer-generated recommendation to the underlying gate’s configuration. Includes optimistic-concurrency check via currentValue — if the gate’s actual current value differs from what the recommendation expected, returns 409 so the caller can refetch and re-decide.