Fix first-turn Agent Node content conflicts

[Yuqing (mydmdm)]

Summary

• suppress the redundant Question sidecar write during canonical node creation and order genuine pre-acknowledgement edits behind the creation command
• reconcile pending Question fields independently so first-prompt content, generated titles, and Agent metadata do not overwrite unrelated local edits
• fence stale content PUT responses after a newer authoritative baseline and document the creation/sync contract

Question 1 remains the current default display label; it is not a revision input or the cause of the conflict. External Utility title generation remains an isolated Job path and required no changes.

Validation

• pnpm --filter @huabu/web test src/store/canvasStore.agentDeltaConflict.test.ts src/store/canvasStore.questionRestore.test.ts src/store/canvasStore/save/__tests__/nodeContentQueue.baseline.test.ts src/hooks/agentTurnController.test.ts
• pnpm --filter @huabu/server test src/modules/canvas/canvas-content-cas.test.ts src/modules/canvas/write-coordinator.test.ts src/modules/canvas/canvas-executor.test.ts
• pnpm format
• pnpm lint:fix (0 errors; existing warnings only)
• pnpm typecheck
• patch applies cleanly to latest fetched origin/alpha (4b83584070671767992757cc88fa9a7698ceec02)

Refs #172