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/docs/API.md

@@ -0,0 +1,24 @@
+# API Documentation
+
+## User Model
+
+### `createUser(data)`
+
+Creates a new user with auto-generated ID and timestamp.
+
+**Parameters:**
+- `data` — Object with `email` and `name` fields
+
+**Returns:** `User` object
+
+**Throws:** `Error` if email is invalid
+
+### `serializeUser(user)`
+
+Serializes a user to JSON string.
+
+## Validation Utilities
+
+- `validateEmail(email)` — Validates email format
+- `validateNotEmpty(value)` — Checks string is not empty
+- `validateMinLength(value, min)` — Checks minimum string length