feat(doctor): brain-home fleet-state check (#1298 follow-up, canon §6)

mosaic-doctor now reports which tree fleet state resolves from and warns
on the drift a launch would hit at runtime:

- stale MOSAIC_BRAIN_HOME (no fleet/agents under it)
- symlinked brain/agents paths (managed-directory boundary)
- group/world-readable agents dir (0700 projection boundary)
- split state: env files in BOTH config-home and brain trees

Resolution mirrors brain-home.ts (#1298): MOSAIC_BRAIN_HOME wins;
canonical ~/.mosaic adopted only at the default config home; else legacy.

test-brain-home-check.sh: extraction-style harness (test-fleet-transport
discipline — functions pulled from the shipped script, never copied), 8
cases covering every branch; wired into test:framework-shell and covered
by the enumeration guard (population 53, all present).

Live-run verified on dragon-lin: caught two real drifts — agents dir
755 (fixed locally) and pre-cutover split state (expected, tracked in
mosaic-brain fleet/SEAT-CUTOVER.md).

packages/mosaic/framework/tools/_scripts/mosaic-doctor

@@ -255,6 +255,68 @@ fleet_declared_transport() {
   printf '%s\n' "${declared:-tmux}"
 }
 
+# Brain-home fleet-state resolution (#1298; canon STRUCTURE-CANON §2).
+#
+# Seat launch envs, roles.local overrides, and profile working copies resolve
+# from the brain home when one is active; roster, baseline roles, run/, and
+# services stay under MOSAIC_HOME. This check surfaces which tree fleet state
+# resolves from and the drift a launch would otherwise hit at runtime:
+#
+# - a stale MOSAIC_BRAIN_HOME pointing at a directory with no fleet/agents is a
+#   misconfiguration the resolver honors (explicit wins) — warn, don't pass;
+# - a symlinked brain or agents dir defeats the managed-directory boundary;
+# - a group/world-readable agents dir violates the 0700 projection boundary;
+# - env files left in the config-home tree while a brain is active are split
+#   state — the write path rejects NEW split writes, but nothing would ever
+#   tell the operator the old files are stranded.
+resolve_brain_home() {
+  local explicit="${MOSAIC_BRAIN_HOME:-}"
+  if [[ -n "$(printf '%s' "$explicit" | tr -d '[:space:]')" ]]; then
+    printf '%s' "$explicit"
+    return
+  fi
+  if [[ "$(cd "$MOSAIC_HOME" 2>/dev/null && pwd -P)" == "$HOME/.config/mosaic" \
+     && -d "$HOME/.mosaic/fleet/agents" ]]; then
+    printf '%s' "$HOME/.mosaic"
+    return
+  fi
+  printf '%s' "$MOSAIC_HOME"
+}
+
+check_brain_home() {
+  local brain agents mode
+  brain="$(resolve_brain_home)"
+
+  if [[ "$brain" == "$MOSAIC_HOME" ]]; then
+    pass "Fleet state home: $MOSAIC_HOME (legacy single-tree; no brain adopted)"
+    return
+  fi
+
+  agents="$brain/fleet/agents"
+  if [[ ! -d "$agents" ]]; then
+    warn "Brain home '$brain' has no fleet/agents — seat envs will not resolve from it. Point MOSAIC_BRAIN_HOME at a brain carrying fleet/agents, or unset it."
+    return
+  fi
+  if [[ -L "$brain" || -L "$agents" ]]; then
+    warn "Brain fleet-state path resolves through a symlink ($brain) — the managed-directory boundary requires regular directories."
+    return
+  fi
+
+  mode="$(stat -c '%a' -- "$agents" 2>/dev/null)" || mode=""
+  if [[ -n "$mode" ]] && (( (8#$mode & 8#077) != 0 )); then
+    warn "Brain agents dir '$agents' is group/world-accessible (mode $mode) — the projection boundary requires 0700."
+    return
+  fi
+
+  if [[ -d "$MOSAIC_HOME/fleet/agents" ]] \
+     && ls "$MOSAIC_HOME/fleet/agents/"*.env* >/dev/null 2>&1; then
+    warn "Fleet env files exist in BOTH trees — brain '$brain' is active but '$MOSAIC_HOME/fleet/agents' still carries env files (split state). Migrate them (mosaic fleet regen) and remove the config-home copies."
+    return
+  fi
+
+  pass "Fleet state home: $brain (brain active); roster + templates: $MOSAIC_HOME"
+}
+
 check_fleet_transport() {
   local transport
   transport="$(fleet_declared_transport)"
@@ -273,6 +335,8 @@ check_fleet_transport() {
 
 check_fleet_transport
 
+check_brain_home
+
 # Legacy migration surfaces should no longer contain symlink trees.
 legacy_paths=(
   "$HOME/.claude/agent-guides"