feat: add preflight validation phase with structured error reporting

- Add preflight activity that validates repo path, config, and credentials before agent execution
- Add formatWorkflowError() with pipe-delimited segments for multi-line log rendering
- Add remediation hints for common failures (auth, billing, config errors)
- Add REPO_NOT_FOUND, AUTH_FAILED, BILLING_ERROR codes with error classification
- Add formatErrorBlock() in WorkflowLogger for indented error display

src/audit/workflow-logger.ts

@@ -311,6 +311,24 @@ export class WorkflowLogger {
     await this.logStream.write(line);
   }
 
+  /**
+   * Format a pipe-delimited error string into indented multi-line display.
+   *
+   * Input:  "phase context|ErrorType|message|Hint: ..."
+   * Output: "Error:       phase context\n             ErrorType\n             ..."
+   */
+  private formatErrorBlock(errorString: string): string {
+    const segments = errorString.split('|');
+    const label = 'Error:       ';
+    const indent = ' '.repeat(label.length);
+
+    const lines = segments.map((segment, i) =>
+      i === 0 ? `${label}${segment.trim()}` : `${indent}${segment.trim()}`
+    );
+
+    return lines.join('\n') + '\n';
+  }
+
   /**
    * Log workflow completion with full summary
    */
@@ -330,7 +348,7 @@ export class WorkflowLogger {
     await this.logStream.write(`Agents:      ${summary.completedAgents.length} completed\n`);
 
     if (summary.error) {
-      await this.logStream.write(`Error:       ${summary.error}\n`);
+      await this.logStream.write(this.formatErrorBlock(summary.error));
     }
 
     await this.logStream.write(`\n`);