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.

docs/plans/CURRENT.md

@@ -51,3 +51,4 @@ git history + Gitea issues.
 - 2026-09-03 — conductor-loop calibration with live collaborator (#43): dispatch via agent-send.sh → receipt → line-by-line diff review → suite-gated integration; docs/TOOLS.md gains Tools (host-side) section + corrected suite counts
 - 2026-09-03 — skill revisions adjudicated (#44): ms-communications integrated as-authored; ms-conductor redraft + conductor remediation (refusal vs outage); TOOLS.md release.sh ensure row
 - 2026-09-03 — M18 seat-role progressive capability restriction (#45; roles resolve to contracts, ceiling ∩ seat grant, fail-closed refusals, roles/researcher.json); task suite 74 → 88
+- 2026-09-03 — M18 follow-up: fail-closed seat resolution under MOSAIC_AGENTS_DIR override (#46, owner decision after live verification); task suite 88 → 90; next action M19