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.
