feat(control): resolve visible agent labels and titles within a window #796

During the external-operator trial, searching C18 returned zero results although C18 was visible. Session records had empty stored titles and no display labels, while the UI showed labels and prompt-derived titles. Two different windows also contained B7, with different providers/conversations. This contributed to the operator selecting the wrong B7.

Evidence: ac_agents_search query C18, call d79e2a4e-5a64-4f1b-91c8-5f578f01a435, returned items [], total 0, complete true, no unavailable windows. Inspected globalCapabilities.ts and workspace/control/agents.ts match stored title, ID, cwd and provider, omitting visible labels and derived titles.

Expected/acceptance: return display label, displayed title, stable ID and window ownership together; support exact window-scoped label lookup and displayed-title search; report both candidates for ambiguous global labels. Keep labels distinct from stable identities. The wrong-target action was an operator mistake, not incorrect MCP routing.

Environment: macOS, development Electron, two Agent Code windows, external MCP plus computer use, observed 2026-09-05. Exact running build revision was not verified. Refs #793, #795.