feat(agent): add llm assistant chat with api tools (task 88)

OpenAI-compatible client with mock fallback, tool-calling loop, and /assistant UI.

Tools map to /api/v1/ endpoints per ADR 0006 direct-tools decision.

src/app/api/agent/chat/route.ts

@@ -0,0 +1,52 @@
+import { z } from "zod";
+import { apiError, apiJson } from "@/lib/api-handler";
+import { resolveApiAuth } from "@/lib/api-auth";
+import { isLlmConfigured } from "@/lib/llm";
+import { runAgentChat } from "@/modules/agent/server/run";
+
+const chatInput = z.object({
+  messages: z
+    .array(
+      z.object({
+        role: z.enum(["user", "assistant"]),
+        content: z.string().trim().min(1).max(8000),
+      }),
+    )
+    .min(1)
+    .max(40),
+});
+
+export async function POST(request: Request) {
+  const auth = await resolveApiAuth(request);
+  if (!auth) {
+    return apiError("Unauthorized", 401);
+  }
+
+  let body: unknown;
+  try {
+    body = await request.json();
+  } catch {
+    return apiError("Invalid JSON body", 400);
+  }
+
+  const parsed = chatInput.safeParse(body);
+  if (!parsed.success) {
+    return apiError(parsed.error.issues[0]?.message ?? "Validation error", 400);
+  }
+
+  try {
+    const result = await runAgentChat({
+      messages: parsed.data.messages,
+      request,
+    });
+
+    return apiJson({
+      ...result,
+      configured: isLlmConfigured(),
+      provider: isLlmConfigured() ? "openai" : "mock",
+    });
+  } catch (err) {
+    const message = err instanceof Error ? err.message : "Agent request failed";
+    return apiError(message, 502);
+  }
+}

src/app/assistant/page.tsx

@@ -0,0 +1,6 @@
+import { AssistantChat } from "@/modules/agent/components/assistant-chat";
+import { isLlmConfigured } from "@/lib/llm";
+
+export default function AssistantPage() {
+  return <AssistantChat configured={isLlmConfigured()} />;
+}