Remove disabled outbound send runtime

[snbafana]

Summary

• remove the disabled RunQueueService outbound send API and scheduler wake hook
• delete the daemon outbound drain, send helpers, Signal echo catchup tracking, and update-shutdown busy state tied to outbound sends
• remove the obsolete outbound drain gate test

Dedup decision

Rechecked the current owners before editing. isOutboundSendEnabled() was still hardcoded false and queueMessageSend had no production callers, so this deletes the dead runtime path in src/runtime/run-queue.ts and src/runtime/daemon/server.ts instead of adding another enablement/config layer. The outbound DB schema and database helpers stay in place for a separate DB/schema decision.

Verification

• rg for removed outbound runtime symbols returned no source hits
• pnpm typecheck
• pnpm check:biome
• pnpm test src/runtime/run-queue.test.ts src/runtime/daemon/server.test.ts src/db/database.test.ts
• commit hook: pnpm check:app-quality
• pre-push hook: pnpm check:ci-local

[snbafana]

Closing this PR per product direction: keep the existing send-message runtime/scaffolding for now. The stack has been rewritten so PR #65 now builds directly on PR #62, and PR #66 builds on the rewritten PR #65.