feat(server): persist node semantic context and retain CAD agent context

Add a semantic_context column to canvas_nodes (schema, sqlc queries,
generated models, repository upserts) and thread SemanticContext through the
domain node, API type, and mappers so CAD-derived context survives canvas
saves.

Treat agent text parts with textSource "cad-context" like settings: keep
them for planning, memory, and image-generation prompts but exclude them
from the persisted user-visible message copy. Add coverage for the CAD
context visibility and mapper behavior.

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

server/internal/logic/mapper.go

@@ -263,6 +263,7 @@ func toAPINodes(items []design.Node) []types.CanvasNode {
 			Status:              item.Status,
 			ParentId:            item.ParentID,
 			LayerRole:           item.LayerRole,
+			SemanticContext:     item.SemanticContext,
 			FontSize:            item.FontSize,
 			FontFamily:          item.FontFamily,
 			FontWeight:          item.FontWeight,
@@ -305,6 +306,7 @@ func toDomainNodes(items []types.CanvasNode) []design.Node {
 			Status:              item.Status,
 			ParentID:            item.ParentId,
 			LayerRole:           item.LayerRole,
+			SemanticContext:     item.SemanticContext,
 			FontSize:            item.FontSize,
 			FontFamily:          item.FontFamily,
 			FontWeight:          item.FontWeight,