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/utils/metrics.ts

@@ -60,7 +60,7 @@ export const costResults: CostResults = {
 };
 
 // Function to display comprehensive timing summary
-export const displayTimingSummary = (): void => {
+const displayTimingSummary = (): void => {
   if (!timingResults.total) {
     console.log(chalk.yellow('No timing data available'));
     return;