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/mock-llm.ts

@@ -0,0 +1,17 @@
+import type { LLMClient } from "../src/llm/llm-client.js";
+
+export function createMockFileClient(purpose = "Test file"): LLMClient {
+	return {
+		async complete() {
+			return `PURPOSE: ${purpose}\nDEPS: none\nCONCEPTS: testing`;
+		},
+	};
+}
+
+export function createMockPackageClient(): LLMClient {
+	return {
+		async complete() {
+			return "ROLE: Test package\nARCH: Test architecture";
+		},
+	};
+}