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/patch.ts

@@ -1,12 +1,12 @@
 import { dirname, join, basename, relative } from "path";
 import { existsSync, readFileSync, writeFileSync } from "fs";
 import { parsePackageMap, renderPackageMap } from "./format.js";
-import { extractFileLLM } from "./llm-extract.js";
-import { extractFileAST } from "./ast-extract.js";
+import { extractFileLLM } from "./llm/llm-extract.js";
+import { extractFileAST } from "./ast/ast-extract.js";
 import { mergeFileData } from "./merge.js";
 import { generateDirectoryMap } from "./init.js";
 import { readdirSync, statSync } from "fs";
-import type { LLMClient } from "./llm-client.js";
+import type { LLMClient } from "./llm/llm-client.js";
 
 const SMALL_PACKAGE_THRESHOLD = 10;