feat(settings): configure default Codex fast mode

[eddieparc]

Summary

• add a persisted /settings toggle for the default OpenAI Codex Fast mode
• enable Fast mode by default for new Codex sessions while preserving /fast as a session override
• cover preference persistence, selector changes, and session reset behavior

Validation

• npx vitest run test/suite/service-tier-settings.test.ts test/suite/service-tier-extension.test.ts test/settings-selector.test.ts
• npm run check
• isolated mock-loop.mjs --run QA (no real provider calls)

Linear: https://linear.app/ublanks/issue/UBL-118/senpi-기본-fast-모드-활성화

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Summary by cubic

Makes OpenAI Codex Fast mode configurable and enabled by default for new sessions. Previously, new sessions reset to the base model; now they start in Fast mode when preferred, selecting the fast catalog variant and using service_tier: "priority".

• Adds openai.defaultFastMode (default: true) to global settings in packages/coding-agent, with SettingsManager.getOpenAIDefaultFastMode() and .setOpenAIDefaultFastMode().
• On session_start, the service-tier extension reads the preference and sets session Fast mode and model (fast or base) for Codex only; non-Codex sessions are unchanged.
• Keeps /fast as a per-session toggle that switches between fast and base variants and updates the session Fast flag.
• Exposes the preference in /settings as “Default Fast mode”.
• Updates tests to cover preference persistence, selector behavior, and session start logic. Linear: UBL-118.

Rollout/Migration

• If you want the old default (start in base mode), set openai.defaultFastMode to false via /settings or in the global settings file.

^{Written for commit 6f7e099. Summary will update on new commits.}