[Cleanup] Consolidate 8 separate gateway maps in UI store into unified registry

## Problem

`ui-store.ts` maintains **8 separate `Record` maps** all indexed by `gatewayId`, each with its own setter action:

```typescript
gatewayStatusMap: Record;
setGatewayStatusByGateway: (gatewayId, status) => void;

gatewayVersionMap: Record;
setGatewayVersion: (gatewayId, version) => void;

gatewayReconnectInfo: Record;
setGatewayReconnectInfo: (gatewayId, info) => void;

gatewayInfoMap: Record;
setGatewayInfoMap: (map) => void;

modelCatalogByGateway: Record;
setModelCatalogForGateway: (gatewayId, models) => void;

agentCatalogByGateway: Record;
setAgentCatalogForGateway: (gatewayId, agents, defaultId) => void;

toolsCatalogByGateway: Record;
setToolsCatalogForGateway: (gatewayId, catalog) => void;

skillsStatusByGateway: Record;
setSkillsStatusForGateway: (gatewayId, report) => void;
```

This is 16 interface members (8 state + 8 setters) for what is logically one concept: per-gateway state.

## Location

**File:** `packages/core/src/stores/ui-store.ts:38-73`

## Fix Approach

Unify into a single gateway registry:

```typescript
interface GatewayState {
status: GatewayConnectionStatus;
version?: string;
reconnectInfo?: { attempt: number; max: number; gaveUp: boolean };
info: GatewayInfo;
models: ModelCatalogEntry[];
agents: { agents: AgentInfo[]; defaultId: string };
tools: ToolsCatalog | null;
skills: SkillStatusReport | null;
}

// In store:
gatewayRegistry: Record;
updateGateway: (gatewayId: string, patch: Partial) => void;
```

16 members → 2 members. Selectors become:
```typescript
const status = useUiStore((s) => s.gatewayRegistry[gatewayId]?.status);
```

**Note:** This is a larger refactor — ~30 files reference the individual maps. Should be done incrementally:
1. Add `gatewayRegistry` + `updateGateway` alongside existing maps
2. Migrate consumers file by file
3. Remove old maps

## Verification

1. Run `pnpm check` — must pass
2. Connect/disconnect gateways, verify all status/catalog/version info displays correctly

## Context

- **WG:** Task & Session Core + UI & Design System
- **Priority:** Medium
- **Estimated effort:** ~2 hours (incremental)