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/img_infinite_canvas.api

@@ -242,6 +242,7 @@ type CanvasNode {
 	Status              string        `json:"status"`
 	ParentId            string        `json:"parentId,optional"`
 	LayerRole           string        `json:"layerRole,optional"`
+	SemanticContext     string        `json:"semanticContext,optional"`
 	FontSize            float64       `json:"fontSize,optional"`
 	FontFamily          string        `json:"fontFamily,optional"`
 	FontWeight          string        `json:"fontWeight,optional"`