feat(agent): implement phase 2 runtime and inline clarification

lib/agent-models.ts

@@ -0,0 +1,77 @@
+export type AgentModelId =
+  | "openai/gpt-5.4-nano"
+  | "openai/gpt-5.4-mini"
+  | "openai/gpt-5.4"
+  | "openai/gpt-5.4-pro";
+
+export type AgentModelMinTier = "starter" | "max";
+export type AgentModelAccessTier = "free" | "starter" | "pro" | "max" | "business";
+
+export interface AgentModel {
+  id: AgentModelId;
+  label: string;
+  minTier: AgentModelMinTier;
+  creditCost: number;
+  description: string;
+}
+
+export const AGENT_MODELS = {
+  "openai/gpt-5.4-nano": {
+    id: "openai/gpt-5.4-nano",
+    label: "GPT-5.4 Nano",
+    minTier: "starter",
+    creditCost: 6,
+    description: "Fastest option for lightweight agent runs",
+  },
+  "openai/gpt-5.4-mini": {
+    id: "openai/gpt-5.4-mini",
+    label: "GPT-5.4 Mini",
+    minTier: "starter",
+    creditCost: 15,
+    description: "Balanced quality and latency for default use",
+  },
+  "openai/gpt-5.4": {
+    id: "openai/gpt-5.4",
+    label: "GPT-5.4",
+    minTier: "starter",
+    creditCost: 38,
+    description: "Higher reasoning quality for complex tasks",
+  },
+  "openai/gpt-5.4-pro": {
+    id: "openai/gpt-5.4-pro",
+    label: "GPT-5.4 Pro",
+    minTier: "max",
+    creditCost: 180,
+    description: "Top-tier capability for hardest workflows",
+  },
+} as const satisfies Record<AgentModelId, AgentModel>;
+
+export const DEFAULT_AGENT_MODEL_ID: AgentModelId = "openai/gpt-5.4-mini";
+
+const AGENT_MODEL_IDS = Object.keys(AGENT_MODELS) as AgentModelId[];
+
+const AGENT_MODEL_TIER_ORDER: Record<AgentModelAccessTier, number> = {
+  free: 0,
+  starter: 1,
+  pro: 2,
+  max: 3,
+  business: 4,
+};
+
+export function getAgentModel(id: string): AgentModel | undefined {
+  return AGENT_MODELS[id as AgentModelId];
+}
+
+export function isAgentModelAvailableForTier(
+  tier: AgentModelAccessTier,
+  modelId: AgentModelId,
+): boolean {
+  const model = AGENT_MODELS[modelId];
+  return AGENT_MODEL_TIER_ORDER[model.minTier] <= AGENT_MODEL_TIER_ORDER[tier];
+}
+
+export function getAvailableAgentModels(tier: AgentModelAccessTier): AgentModel[] {
+  return AGENT_MODEL_IDS.map((id) => AGENT_MODELS[id]).filter((model) =>
+    isAgentModelAvailableForTier(tier, model.id),
+  );
+}