refactor: remove ~750 lines of dead code across 12 files

- Delete 4 dead files: pre-recon.ts, tool-checker.ts, input-validator.ts, environment.ts
- Remove runClaudePromptWithRetry() and its now-unused imports from claude-executor.ts
- De-export unused symbols: AGENT_ORDER, getParallelGroups, logError, isRouterMode, showHelp, displayTimingSummary
- De-export unused types: ProcessingState, ProcessingResult, SdkMessage, MessageDispatchResult, MessageDispatchContext
- Remove dead import (path from zx) in session-manager.ts and deprecated comment in config.ts

src/error-handling.ts

@@ -42,8 +42,7 @@ export class PentestError extends Error {
   }
 }
 
-// Centralized error logging function
-export async function logError(
+async function logError(
   error: Error & { type?: PentestErrorType; retryable?: boolean; context?: PentestErrorContext },
   contextMsg: string,
   sourceDir: string | null = null