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/agent.sh

@@ -57,6 +57,14 @@ fi
 # The seat's SOUL.md overrides the contract persona; governance contracts
 # are never overridden.
 AGENTS_DIR="${MOSAIC_AGENTS_DIR:-agents}"
+# Fail closed (owner decision, #46): an explicit agents-dir override that
+# cannot resolve the named seat refuses the launch - a seatless launch has
+# no identity, no SOUL, and no role ceiling to bind. Unset the override for
+# the M13 plain governed TUI.
+if [ -n "${MOSAIC_AGENTS_DIR:-}" ] && [ ! -f "$AGENTS_DIR/$NAME/agent.json" ]; then
+    echo "agent: no seat definition for '$NAME' in overridden agents dir: $AGENTS_DIR (unset MOSAIC_AGENTS_DIR for a plain governed TUI)" >&2
+    exit 4
+fi
 ROLE=""
 DEFCAPS=""
 AGENT_DEF_SKILLS=""