fix(repo-selector): preserve results while searching

[ryans-posthog]

Summary

• The repository selector flashed empty between keystrokes because each new search query key swapped the multi-query state to pending with no data.
• Pass placeholderData: (prev) => prev to the per-integration useGithubRepositories query so React Query keeps showing the prior page while the next search loads.
• First-open "Loading repositories..." state is unchanged (no prior data to fall back to).

Test plan

• Open the repository selector and type a query — the existing list should remain visible while new results load instead of collapsing to empty.
• Confirm the initial "Loading repositories..." state still shows on first open.
• pnpm --filter code typecheck passes.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(repo-selector): preserve results whi..." | Re-trigger Greptile}