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On a host with ~/.mosaic but no ~/.mosaic/fleet/agents, the implicit resolution path passes silently with output byte-identical to a host with no brain at all. fred traced why this is the shape web1 will be in (#1288 comment 23133): nothing in product code ever creates ~/.mosaic/fleet/agents — the first fleet write resolves BEFORE creating (generated-env-boundary), decides legacy, then manufactures the evidence that keeps the host legacy. The three operator checks all agree and all point the wrong way; the doctor is the only one that can disagree. check_brain_home now emits a [NOTE] (not a WARN — nothing is broken yet) when ~/.mosaic exists at the default config home without fleet/agents, telling the operator the first 'mosaic fleet regen' locks in the legacy tree. Explicit MOSAIC_BRAIN_HOME misconfiguration still warns (behavior unchanged); custom homes stay silent (hermetic). test-brain-home-check.sh: +3 cases (greenfield note, no-mosaic pass, stray-mosaic-at-custom-home silence) — 11 total.