feat(FN-3939): audit and patch PR #59 residual defects, add retry parity an

Completes FN-3939 audit work for PR #59 residual defects by refactoring the mission execution loop in the engine (halving its complexity), adding self-healing test coverage, and wiring in task workflow route improvements. Retry and validation behavior updates are documented in the changeset and skil

Fusion-Task-Id: FN-3939

docs/missions.md

@@ -216,6 +216,8 @@ On task completion, the scheduler calls `MissionExecutionLoop.processTaskOutcome
 3. Fire AI validator agent against contract assertions
 4. Record `MissionValidatorRun` with per-assertion results
 
+Validation runs are internal mission-loop operations: Fusion does **not** create visible `🔍 Validate:` board tasks for single-feature validation.
+
 ```typescript
 interface MissionValidatorRun {
   id: string;