fix: isolate scratchpad session working directories

Give each scratchpad session its own working directory, migrate existing scratchpad sessions on workspace load, and route sidecar turns through the session-specific cwd. Add regression coverage for create, duplicate, delete, and migration flows, and document the new persistence model.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/shared/domain/session.ts

@@ -48,6 +48,7 @@ export interface SessionRecord {
   isPinned?: boolean;
   isArchived?: boolean;
   interactionMode?: InteractionMode;
+  cwd?: string;
   messages: ChatMessageRecord[];
   lastError?: string;
   sessionModelConfig?: SessionModelConfig;