[codex] Cache convoy list refreshes

[Spectralgo]

Summary

• Add a main-process SWR/singleflight cache for gastown.convoys.list keyed by normalized list input.
• Return the last successful convoy list immediately while stale refreshes run in the background.
• Add focused router tests for cold duplicate sharing and stale data during refresh.

Performance Note

This masks the 26s gt convoy list --json stall after the first successful load by serving cached convoy data immediately. Optimizing the upstream gt command remains separate.

Validation

• bun test apps/desktop/src/lib/trpc/routers/gastown/convoys.test.ts
• bun run --cwd apps/desktop typecheck
• ./node_modules/.bin/biome check apps/desktop/src/lib/trpc/routers/gastown/convoys.ts apps/desktop/src/lib/trpc/routers/gastown/convoys.test.ts

[github-actions]

🧹 Preview Cleanup Complete

The following preview resources have been cleaned up:

• ⚠️ Neon database branch
• ⚠️ Electric Fly.io app

Thank you for your contribution! 🎉