feat: M7 proper LLM integration with dual providers, caching, and parallel batching

- Add LLM client abstraction (src/llm-client.ts) with factory pattern
- Add OpenAI-compatible external client (src/external-llm-client.ts)
- Add Kimi.com client using Anthropic-based API (src/kimi-llm-client.ts)
- Add Pi native LLM stub (src/pi-llm-client.ts) for future ExtensionAPI wiring
- Add SHA-256 disk cache at ~/.cache/pi-project-map/ (src/llm-cache.ts)
- Add parallel batching with p-limit, retry + exponential backoff (src/llm-batch.ts)
- Rewrite llm-extract.ts to use real LLM calls with structured prompts
  - File-level: PURPOSE, DEPS, CONCEPTS
  - Package-level: ROLE, ARCH
  - Context truncation, 50KB skip, cache before LLM call
- Wire CLI with --llm-provider, --llm-model, --llm-base-url flags
- Update config.ts with llmProvider, llmBaseUrl fields
- Update init.ts and patch.ts to accept optional LLMClient
- Add sample project fixture for manual testing
- Add tests: llm-cache (3), llm-batch (5), llm-integration (8 with real Kimi API),
  pi-extension (14 mocked)
- All 56 tests pass

fixtures/sample-project/src/models/user.ts

@@ -0,0 +1,23 @@
+import { validateEmail } from "../utils/validation.js";
+
+export interface User {
+  id: string;
+  email: string;
+  name: string;
+  createdAt: Date;
+}
+
+export function createUser(data: Omit<User, "id" | "createdAt">): User {
+  if (!validateEmail(data.email)) {
+    throw new Error("Invalid email");
+  }
+  return {
+    id: crypto.randomUUID(),
+    ...data,
+    createdAt: new Date(),
+  };
+}
+
+export function serializeUser(user: User): string {
+  return JSON.stringify(user);
+}