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/lib/llm/openai-compatible.ts

@@ -0,0 +1,74 @@
+import type { ChatCompletionRequest, ChatCompletionResult, LlmClient } from "./types";
+
+type OpenAiMessage = {
+  role: string;
+  content: string | null;
+  tool_calls?: Array<{
+    id: string;
+    type: "function";
+    function: { name: string; arguments: string };
+  }>;
+  tool_call_id?: string;
+  name?: string;
+};
+
+export function createOpenAiCompatibleClient(options: {
+  baseUrl: string;
+  apiKey: string | null;
+  model: string;
+}): LlmClient {
+  const completionsUrl = `${options.baseUrl.replace(/\/$/, "")}/chat/completions`;
+
+  return {
+    async chatCompletion(request: ChatCompletionRequest): Promise<ChatCompletionResult> {
+      const headers: Record<string, string> = {
+        "Content-Type": "application/json",
+      };
+      if (options.apiKey) {
+        headers.Authorization = `Bearer ${options.apiKey}`;
+      }
+
+      const body = {
+        model: options.model,
+        messages: request.messages as OpenAiMessage[],
+        tools: request.tools,
+        tool_choice: request.tools?.length ? "auto" : undefined,
+      };
+
+      const response = await fetch(completionsUrl, {
+        method: "POST",
+        headers,
+        body: JSON.stringify(body),
+      });
+
+      if (!response.ok) {
+        const detail = await response.text();
+        throw new Error(`LLM request failed (${response.status}): ${detail.slice(0, 400)}`);
+      }
+
+      const payload = (await response.json()) as {
+        choices?: Array<{
+          finish_reason?: string | null;
+          message?: OpenAiMessage;
+        }>;
+      };
+
+      const choice = payload.choices?.[0];
+      const message = choice?.message;
+      if (!message) {
+        throw new Error("LLM response missing message");
+      }
+
+      return {
+        message: {
+          role: message.role as ChatCompletionResult["message"]["role"],
+          content: message.content,
+          tool_calls: message.tool_calls,
+          tool_call_id: message.tool_call_id,
+          name: message.name,
+        },
+        finishReason: choice.finish_reason ?? null,
+      };
+    },
+  };
+}