feat(gcp): Pub/Sub in Cloud Explorer

[hectorvent]

Add Pub/Sub to the Cloud Explorer (gcp)

The gcp emulator implements Pub/Sub (~0 operations). The UI does not surface it yet.

Backend (packages/api)

• src/cloud-spi/messagingSchema.ts — messagingSchemaFor('gcp') (model: src/cloud-spi/storageSchema.ts)
• src/adapter-gcp/GcpPubSubAdapter.ts implements CloudServiceAdapter + GcpPubSubAdapter.test.ts (model: src/adapter-aws/AwsStorageAdapter.ts)
• Register new GcpPubSubAdapter() in src/cloudProxy.ts; add services.push({...}) + schema() fallback in src/service/CloudProxyService.ts

Frontend (packages/frontend)

• Verify nav shows Pub/Sub for gcp and normalizeService() handles the literal (pages/CloudExplorerPage.tsx)
• Confirm generic DynamicResourceView rendering; add a service === '...' panel only if deep UX is needed

Acceptance

• Pub/Sub appears in the gcp nav; list / create / delete / inspect work against the local emulator
• Adapter unit test passes

Depends on #66 (new messaging service type).
Emulator source: floci-gcp/src/main/java/io/floci/gcp/services/pubsub/