refactor: remove ~275 lines of dead code and enable stricter tsconfig

- Delete unused src/cli/ui.ts, remove zod dependency, drop 4 dead functions (logError, handleToolError, getRetryDelay, displayTimingSummary)
- Remove 8 unused types/interfaces and 3 duplicate formatting utils from audit/utils.ts
- Narrow export surface: make 7 message-handler functions private, remove unused audit re-exports, unexport AgentDefinition and path constants
- Remove unused runClaudePrompt params (sessionMetadata, attemptNumber) and update caller
- Enable tsconfig noUnusedLocals, noUnusedParameters, noImplicitReturns, noImplicitOverride, noFallthroughCasesInSwitch

src/audit/index.ts

@@ -17,7 +17,3 @@
  */
 
 export { AuditSession } from './audit-session.js';
-export { AgentLogger } from './logger.js';
-export { WorkflowLogger } from './workflow-logger.js';
-export { MetricsTracker } from './metrics-tracker.js';
-export * as AuditUtils from './utils.js';

src/audit/utils.ts

@@ -19,8 +19,8 @@ const __filename = fileURLToPath(import.meta.url);
 const __dirname = path.dirname(__filename);
 
 // Get Shannon repository root
-export const SHANNON_ROOT = path.resolve(__dirname, '..', '..');
-export const AUDIT_LOGS_DIR = path.join(SHANNON_ROOT, 'audit-logs');
+const SHANNON_ROOT = path.resolve(__dirname, '..', '..');
+const AUDIT_LOGS_DIR = path.join(SHANNON_ROOT, 'audit-logs');
 
 export interface SessionMetadata {
   id: string;
@@ -132,39 +132,6 @@ export async function atomicWrite(filePath: string, data: object | string): Prom
   }
 }
 
-/**
- * Format duration in milliseconds to human-readable string
- */
-export function formatDuration(ms: number): string {
-  if (ms < 1000) {
-    return `${ms}ms`;
-  }
-
-  const seconds = ms / 1000;
-  if (seconds < 60) {
-    return `${seconds.toFixed(1)}s`;
-  }
-
-  const minutes = Math.floor(seconds / 60);
-  const remainingSeconds = Math.floor(seconds % 60);
-  return `${minutes}m ${remainingSeconds}s`;
-}
-
-/**
- * Format timestamp to ISO 8601 string
- */
-export function formatTimestamp(timestamp: number = Date.now()): string {
-  return new Date(timestamp).toISOString();
-}
-
-/**
- * Calculate percentage
- */
-export function calculatePercentage(part: number, total: number): number {
-  if (total === 0) return 0;
-  return (part / total) * 100;
-}
-
 /**
  * Read and parse JSON file
  */