fix(doctor): greenfield brain lock-in note (#1288, fred's finding 1)

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.

packages/mosaic/framework/tools/_scripts/test-brain-home-check.sh

@@ -39,22 +39,26 @@ eval "$(extract_function check_brain_home)"
 ROOT=$(mktemp -d)
 trap 'rm -rf "$ROOT"' EXIT
 
+# note output is neither [OK] nor [WARN] — assert it directly in the case below.
 run_case() {
-  # label, expect (ok|warn), then env assignments as arguments.
+  # label, expect (ok|warn|note), then env assignments as arguments.
   # The check runs under `env` in a subshell, so its warn() also prints a
   # sentinel the parent counts — a subshell counter would never be visible.
   local label="$1" expect="$2"
   shift 2
-  local out warns
-  out=$(env "$@" bash -c "warn() { echo \"[WARN] \$*\"; }; pass() { echo \"[OK] \$*\"; return 0; }; $(extract_function resolve_brain_home); $(extract_function check_brain_home); check_brain_home" 2>&1)
+  local out warns notes
+  out=$(env "$@" bash -c "warn() { echo \"[WARN] \$*\"; }; note() { echo \"[NOTE] \$*\"; return 0; }; pass() { echo \"[OK] \$*\"; return 0; }; $(extract_function resolve_brain_home); $(extract_function check_brain_home); check_brain_home" 2>&1)
   warns=$(printf '%s\n' "$out" | grep -c '^\[WARN\]' || true)
-  if [[ "$expect" == ok && "$warns" -eq 0 ]]; then
+  notes=$(printf '%s\n' "$out" | grep -c '^\[NOTE\]' || true)
+  if [[ "$expect" == ok && "$warns" -eq 0 && "$notes" -eq 0 ]]; then
     echo "ok - $label"
   elif [[ "$expect" == warn && "$warns" -gt 0 ]]; then
     echo "ok - $label (warned)"
+  elif [[ "$expect" == note && "$notes" -gt 0 ]]; then
+    echo "ok - $label (noted)"
   else
     echo "output: $out" >&2
-    fail "$label: expected $expect (warns=$warns)"
+    fail "$label: expected $expect (warns=$warns notes=$notes)"
   fi
 }
 
@@ -105,4 +109,19 @@ chmod 700 "$ROOT/brain-clean/fleet/agents" "$ROOT/config6/fleet/agents"
 run_case "empty config-home agents dir alongside brain passes" ok \
   MOSAIC_HOME="$ROOT/config6" HOME="$ROOT" MOSAIC_BRAIN_HOME="$ROOT/brain-clean"
 
+# ── greenfield brain-without-agents at the default home → note (#1288) ─────
+mkdir -p "$ROOT/gf-home/.config/mosaic/fleet" "$ROOT/gf-home/.mosaic"
+run_case "~/.mosaic without fleet/agents at default home notes the lock-in" note \
+  MOSAIC_HOME="$ROOT/gf-home/.config/mosaic" HOME="$ROOT/gf-home"
+
+# ── no ~/.mosaic at all at the default home → clean pass ─────────────────
+mkdir -p "$ROOT/plain-home/.config/mosaic/fleet"
+run_case "no ~/.mosaic at default home passes silently" ok \
+  MOSAIC_HOME="$ROOT/plain-home/.config/mosaic" HOME="$ROOT/plain-home"
+
+# ── custom (non-default) home with a stray ~/.mosaic → still silent ──────
+mkdir -p "$ROOT/custom-home/fleet/agents" "$ROOT/custom-home/.mosaic"
+run_case "custom home with stray ~/.mosaic stays silent" ok \
+  MOSAIC_HOME="$ROOT/custom-home" HOME="$ROOT/custom-home"
+
 echo "ok - mosaic-doctor brain-home check"