chore(ts): migrate use-runtime-adapter to TypeScript

[oxyc]

PR 4 of 4 for #41 — the big one (~1500 LOC, the largest single migration). No behaviour change; pure type addition over the existing logic. Wires in the shared RuntimeEvent, UiMessage, WireMessage, UiContentPart, WireContentBlock, and SessionUsageSnapshot types from PR 3 (#49).

Hook signature

useAssistantRuntime() now returns a typed AssistantRuntimeHandle:

interface AssistantRuntimeHandle {
  runtime: ReturnType<typeof useExternalStoreRuntime>;
  loadConversation: (uuid: string) => Promise<void>;
  approveToolCall: (options?: { trustHost?: boolean }) => void;
  denyToolCall: () => void;
  pendingApprovals: PendingApproval[];
  undoableActions: Record<string, UndoableActionState>;
  undoAction: (toolCallId: string, auditId: number) => Promise<void>;
  retryToolCall: (toolCallId: string) => Promise<void>;
  retryingIds: Set<string>;
}

window.gdsAssistant

window.gdsAssistant is now declared via declare global with the GdsAssistantGlobal interface: localised config (restUrl, restBase, nonce, modelPricing) plus the two hooks we attach ourselves (sendChatMessage, openChat). No more silent as any access.

SSE event narrowing

The big for await (const event of parseSSE(...)) switch now narrows on event.type against the discriminated RuntimeEvent union. Each case's event.data is the precise per-event shape — event.data.input_tokens, event.data.tool_use_id, event.data.trustable_host etc. all resolve to real types. parseSSE itself returns AsyncGenerator<RuntimeEvent> rather than a loose {type, data} stream.

Type interop boundary

assistant-ui's ExternalStoreAdapter types onNew/onEdit with its own (narrower) AppendMessage shape. Our adapter accepts a superset (control messages, client-tool-result resumes, programmatic retry from a text string), so we cast at the useExternalStoreRuntime(adapter as …) boundary rather than weakening the internal OnNewMessage type. Comment explains why.

Consumers

app.jsx, components/Composer.jsx, components/SidePanels.jsx, components/Messages.jsx, components/Thread.jsx all import without an extension — webpack + TS resolve to the .ts file automatically. No caller changes needed in this PR.

Verification

• npm run typecheck — clean
• npm run build — bundle includes useAssistantRuntime, loadConversation, approveToolCall
• npm run test:unit — 17/17 pass (restorePendingApprovalsFromHistory test still passes with its existing inputs)
• npm run lint:js — only the pre-existing no-nested-ternary (line 973) warning carried over from the JS source

Closes #41

• PR 1 (chore(ts): enable TypeScript toolchain + migrate UndoContext as smoke test #47): setup + UndoContext smoke ✓ merged
• PR 2 (chore(ts): migrate editor-bridge to TypeScript #48): editor-bridge.ts ✓ merged
• PR 3 (chore(ts): add shared types/runtime.ts module #49): types/runtime.ts shared module ✓ merged
• PR 4 (this): use-runtime-adapter.ts

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