feat: add gateway diagnostics readiness

[rogerchappel]

Summary

• add additive gateway diagnostics readiness signals per runtime
• report gateway URL/token presence, stored vs ephemeral device identity, connection state, and blockers
• cover readiness behavior in the existing gateway diagnostics route test

Verification

• git diff --check
• pnpm test src/app/api/openclaw/gateway/diagnostics/route.test.ts
• pnpm exec eslint src/lib/gateway-chat-pool.ts src/app/api/openclaw/gateway/diagnostics/route.test.ts
• pnpm exec tsc --noEmit --pretty false
• push hook: pnpm typecheck && pnpm build

Risk

Low. This is additive read-only diagnostics data and does not probe gateways, mutate config, change runtime routing, or alter session behavior.