feat: complete working-copies workspace-first cleanup

- Remove clone_mode/branch/new_branch from frontend create session flow.
- Add workspace picker to CreateSessionForm; auto-create default workspace when repo selected.
- Fix tool-starter.tsx and use-start-tool.ts createInstance signatures after API change.
- Remove clone mode badge from SessionCard.
- Delete stale backend unit tests referencing removed clone_mode schema fields.
- Update OpenSpec working-copies tasks and mark change completed.
- Regenerate project maps.

Quality gates: npm run typecheck, npm run lint, npm test -- --run (82 passed),
python3 -m py_compile on changed backend files.

apps/web/src/components/features/session/session-card.tsx

@@ -176,14 +176,6 @@ export function SessionCard({
 						)}{" "}
 						· {session.tool_type_name}
 					</p>
-					{session.clone_mode && (
-						<p className="muted session-card-meta">
-							<Icon name="branch" size="sm" />
-							{session.clone_mode === "clone"
-								? `Clone${session.branch ? ` (${session.branch})` : ""}`
-								: "Mount"}
-						</p>
-					)}
 					{session.url && (
 						<p className="session-card-url">
 							<button