feat(sandcastle): add roundHistory to LoopResult and plannerOutput to TaskSpec

feat(sandcastle): add roundHistory to LoopResult and plannerOutput to TaskSpec

Replace the unused onRoundComplete callback with a structured
roundHistory array that accumulates RoundSnapshot per round
(including post-loop validation retry). Attach raw planner stdout
to TaskSpec.plannerOutput for downstream verification use.

This enables a future planner-verification step to receive the full
findings history alongside the original plan context.