feat: remove heuristics, go LLM-only with mocks + restructure

BREAKING: Heuristic fallback removed — LLM client now required

Changes:
- Remove extractFileHeuristic, extractPackageHeuristic, all helpers
- extractFileLLM / extractPackageLLM now throw LLMError when client missing
- Add hybrid binary detection: extension blacklist + content sniffing
- Increase file size limit: 50KB → 500KB (text files only)
- Restructure src/ into subdirectories:
  - src/llm/ — all LLM clients, extract, batch, cache, error
  - src/ast/ — AST extraction
  - src/cli/ — CLI entry point
- Update all imports across codebase and tests
- Add tests/mock-llm.ts helper for deterministic mock clients
- Update all tests to use mock LLM clients (no heuristics dependency)
- All 52 tests passing (including 8 real LLM integration tests)

tests/llm-cache.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect, beforeEach, afterEach } from "vitest";
-import { getCached, setCached } from "../src/llm-cache.js";
+import { getCached, setCached } from "../src/llm/llm-cache.js";
 import { existsSync, unlinkSync } from "fs";
 import { join } from "path";
 import { tmpdir } from "os";