FN-5941: stop missions stalling on incompatible/custom-role agents

Importing a catalog ("company") agent assigns role "custom", which the
scheduler never auto-assigns mission/queue work to. Combined with a
model/provider that rejects the "developer" system role, this surfaced as
an invisible, repeating failure loop (GitHub #1261).

- pi.ts: treat an unsupported message-role rejection as a model-selection
  error so a configured fallback model is tried once (single-swap guarded)
  before the task is marked failed.
- mission-autopilot.ts: block a mission feature immediately on an
  operator-actionable failure instead of burning the retry budget
  re-running the same cryptic error.
- mission-routes.ts: preflight mission start — when ephemeral agents are
  disabled and no eligible executor exists, fail fast with an actionable
  message instead of queueing tasks forever.
- agent import route + AgentImportModal: warn when only custom-role agents
  are imported and no executor exists.
- agent-assignment.ts: extract shared listEligibleExecutorAgents helper.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/engine/src/index.ts

@@ -119,6 +119,7 @@ export {
   type InteractiveAgentResult,
   type InteractiveAgentFactory,
 } from "./interactive-ai-session.js";
+export { selectPermanentAgentForTask, listEligibleExecutorAgents } from "./agent-assignment.js";
 
 // Register createFnAgent into core's loader so consumers in @fusion/core
 // (e.g. ai-summarize, memory-compaction) can resolve it without a circular