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)

src/init.ts

@@ -4,13 +4,13 @@ import {
 	type PackageMapData,
 	type FileEntry,
 } from "./format.js";
-import { extractFileLLM, extractPackageLLM } from "./llm-extract.js";
-import { extractFileAST } from "./ast-extract.js";
+import { extractFileLLM, extractPackageLLM } from "./llm/llm-extract.js";
+import { extractFileAST } from "./ast/ast-extract.js";
 import { mergeFileData } from "./merge.js";
-import { processFiles } from "./llm-batch.js";
+import { processFiles } from "./llm/llm-batch.js";
 import { writeFileSync } from "fs";
 import { join } from "path";
-import type { LLMClient } from "./llm-client.js";
+import type { LLMClient } from "./llm/llm-client.js";
 
 export interface InitOptions {
 	verbose?: boolean;