Fix RunPane background session focus

[parsakhaz]

Summary

• Fixes the RunPane background pane focus path where agent-created panes could still switch the visible active Pane UI pane.
• Threads an activateOnCreate hint through task-queue session creation and session:created renderer handling.
• Keeps default app-created sessions activating normally while preserving the current active pane for background RunPane creation.

Fixes #282.

Tests

• pnpm --filter main exec vitest run src/ipc/runpane.test.ts --reporter=dot
• pnpm --filter frontend exec vitest run src/stores/sessionStore.test.ts --reporter=dot
• pnpm --filter main typecheck
• pnpm --filter frontend typecheck
• git diff --check

Notes

No merge, deploy, or release has been performed. This is ready for manual review.