feat(agent): fail-closed seat resolution under MOSAIC_AGENTS_DIR override (#46)

Owner decision after live verification of M18: an explicit agents-dir
override that cannot resolve the named seat now refuses the launch
(exit 4, names the seat and dir) instead of launching seatless and
unbounded. Unsetting the override keeps the M13 plain governed TUI.
MOSAIC_ROLES_DIR needs no symmetric change - the M18 gate already
refuses unresolvable role contracts.

Task suite 88 -> 90 (refusal + refusal-names-the-seat). TOOLS.md Agent
section documents the refusal.

Suites 24/90/14/17 + verify green.

scripts/test-task.sh

@@ -299,6 +299,20 @@ EOF
     grep -q "nothing in common -> tool-free seat" "$SANDBOX/roleseat-stderr.txt" 2>/dev/null \
         && check "tool-free outcome recorded loudly" 0 || check "tool-free outcome recorded loudly" 1
 
+    # fail-closed seat resolution (#46, owner decision): an explicit
+    # agents-dir override that cannot resolve the named seat refuses.
+    env MOSAIC_CONFIG="$SANDBOX/mock-config.json" MOSAIC_AGENTS_DIR="$SANDBOX/agents" \
+        scripts/agent.sh nobody </dev/null >"$SANDBOX/nobody-stdout.txt" 2>"$SANDBOX/nobody-stderr.txt"
+    RC=$?
+    if [ "$RC" -eq 4 ]; then
+        PASS=$((PASS+1)); echo "${C_OK}OK${C_RESET}    overridden agents dir without seat definition refuses (exit 4)"
+    else
+        FAIL=$((FAIL+1)); echo "${C_FAIL}FAIL${C_RESET} overridden agents dir without seat definition refuses (exit $RC)" >&2
+        echo "NOBODY stderr:" >&2; cat "$SANDBOX/nobody-stderr.txt" >&2
+    fi
+    grep -q "no seat definition for 'nobody'" "$SANDBOX/nobody-stderr.txt" 2>/dev/null \
+        && check "seat-resolution refusal names the seat" 0 || check "seat-resolution refusal names the seat" 1
+
     # capability policy (M9): least-privilege intersection
     POL="$SANDBOX/data/workspaces"; mkdir -p "$POL"
     pol_run() { # missionTools(ABSENT|json) taskTools(ABSENT|json) -> stderr MOSAIC_TOOLS value