Codex was clearing fast mode after approving a plan.
This keeps fast mode and plan mode independent. Approving a plan still exits plan collaboration mode, and the follow-up implementation turn now keeps serviceTier=fast when fast mode was already enabled.
Verified locally:
- node node_modules/vitest/vitest.mjs run packages/server/src/server/agent/providers/codex-app-server-agent.test.ts --bail=1
- node node_modules/vitest/vitest.mjs run packages/server/src/server/agent/agent-manager.test.ts --bail=1
- npm run typecheck
