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/mock.ts

@@ -0,0 +1,94 @@
+import type { ChatCompletionRequest, ChatCompletionResult, LlmClient } from "./types";
+
+function lastUserText(messages: ChatCompletionRequest["messages"]): string {
+  for (let i = messages.length - 1; i >= 0; i -= 1) {
+    const message = messages[i];
+    if (message?.role === "user" && message.content) {
+      return message.content.toLowerCase();
+    }
+  }
+  return "";
+}
+
+function hadToolResults(messages: ChatCompletionRequest["messages"]): boolean {
+  return messages.some((message) => message.role === "tool");
+}
+
+export function createMockLlmClient(): LlmClient {
+  return {
+    async chatCompletion(request: ChatCompletionRequest): Promise<ChatCompletionResult> {
+      const userText = lastUserText(request.messages);
+
+      if (hadToolResults(request.messages)) {
+        return {
+          message: {
+            role: "assistant",
+            content: "Done — I updated your household data using the API.",
+          },
+          finishReason: "stop",
+        };
+      }
+
+      if (userText.includes("milk") && request.tools?.length) {
+        return {
+          message: {
+            role: "assistant",
+            content: null,
+            tool_calls: [
+              {
+                id: "mock_call_add_item",
+                type: "function",
+                function: {
+                  name: "add_list_item",
+                  arguments: JSON.stringify({
+                    listType: "shopping",
+                    text: "milk",
+                  }),
+                },
+              },
+            ],
+          },
+          finishReason: "tool_calls",
+        };
+      }
+
+      if (userText.includes("calendar") && request.tools?.length) {
+        const now = new Date();
+        const from = new Date(now);
+        from.setDate(from.getDate() - 1);
+        const to = new Date(now);
+        to.setDate(to.getDate() + 7);
+
+        return {
+          message: {
+            role: "assistant",
+            content: null,
+            tool_calls: [
+              {
+                id: "mock_call_list_events",
+                type: "function",
+                function: {
+                  name: "list_events",
+                  arguments: JSON.stringify({
+                    from: from.toISOString(),
+                    to: to.toISOString(),
+                  }),
+                },
+              },
+            ],
+          },
+          finishReason: "tool_calls",
+        };
+      }
+
+      return {
+        message: {
+          role: "assistant",
+          content:
+            "I'm the famapp assistant (mock provider). Ask me to add items or check the calendar.",
+        },
+        finishReason: "stop",
+      };
+    },
+  };
+}