fix: Map priority mode to fast

[benbrandt]

Closes #285

[Copilot]

Pull request overview

Fixes an ACP fast-mode regression where Codex normalizes the configured fast tier to the canonical priority tier, causing codex-acp to incorrectly initialize Fast mode as disabled and then clear the session tier on first prompt.

Changes:

• Treat serviceTier: "priority" as Fast mode enabled when initializing/resuming a session (same as "fast").
• Introduce a small helper (serviceTierEnablesFast) and a widened tier type (SupportedServiceTier) to represent Codex’s canonical tier value.
• Add a Vitest case ensuring a priority session tier results in serviceTier: "fast" being sent on the first turn.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/FastModeConfig.ts	Adds SupportedServiceTier and serviceTierEnablesFast() to treat "priority" as enabling Fast mode.
src/CodexAcpServer.ts	Uses serviceTierEnablesFast() to initialize fastModeEnabled for new/resumed sessions.
src/CodexAcpClient.ts	Stores response.serviceTier as SupportedServiceTier in SessionMetadata to preserve "priority".
src/tests/CodexACPAgent/fast-mode-config.test.ts	Adds coverage verifying "priority" initializes Fast mode on and triggers serviceTier: "fast" on prompt.

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