feat: add type-safe error handling utilities

- Add isError() type guard to check if value is Error instance
- Add getErrorMessage() utility for safe error message extraction
  in catch blocks (handles TypeScript 4.4+ unknown error type)
- Replace all (err as Error).message patterns with getErrorMessage(err)
  across server.ts, cli.ts, ralph-loop.ts, and screen-manager.ts
- Follows TypeScript best practice of treating caught errors as unknown

This improves code safety by properly handling the case where caught
values may not be Error instances (e.g., thrown strings or objects).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/web/server.ts

@@ -23,19 +23,20 @@ import { ScreenManager } from '../screen-manager.js';
 import { getStore } from '../state-store.js';
 import { generateClaudeMd } from '../templates/claude-md.js';
 import { v4 as uuidv4 } from 'uuid';
-import type {
-  CreateSessionRequest,
-  RunPromptRequest,
-  SessionInputRequest,
-  ResizeRequest,
-  CreateCaseRequest,
-  QuickStartRequest,
-  CreateScheduledRunRequest,
-  QuickRunRequest,
-  ApiResponse,
-  SessionResponse,
-  QuickStartResponse,
-  CaseInfo,
+import {
+  getErrorMessage,
+  type CreateSessionRequest,
+  type RunPromptRequest,
+  type SessionInputRequest,
+  type ResizeRequest,
+  type CreateCaseRequest,
+  type QuickStartRequest,
+  type CreateScheduledRunRequest,
+  type QuickRunRequest,
+  type ApiResponse,
+  type SessionResponse,
+  type QuickStartResponse,
+  type CaseInfo,
 } from '../types.js';
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
@@ -376,7 +377,7 @@ export class WebServer extends EventEmitter {
         this.broadcast('session:updated', { session: session.toDetailedState() });
         return { success: true, message: 'Interactive session started' };
       } catch (err) {
-        return { error: (err as Error).message };
+        return { error: getErrorMessage(err) };
       }
     });
 
@@ -399,7 +400,7 @@ export class WebServer extends EventEmitter {
         this.broadcast('session:updated', { session: session.toDetailedState() });
         return { success: true, message: 'Shell session started' };
       } catch (err) {
-        return { error: (err as Error).message };
+        return { error: getErrorMessage(err) };
       }
     });
 
@@ -580,7 +581,7 @@ export class WebServer extends EventEmitter {
           respawnStatus: controller.getStatus(),
         };
       } catch (err) {
-        return { error: (err as Error).message };
+        return { error: getErrorMessage(err) };
       }
     });
 
@@ -694,7 +695,7 @@ export class WebServer extends EventEmitter {
       } catch (err) {
         // Clean up session on error too
         await this.cleanupSession(session.id);
-        return { success: false, sessionId: session.id, error: (err as Error).message };
+        return { success: false, sessionId: session.id, error: getErrorMessage(err) };
       }
     });
 
@@ -809,7 +810,7 @@ export class WebServer extends EventEmitter {
 
         return { success: true, case: { name, path: casePath } };
       } catch (err) {
-        return { success: false, error: (err as Error).message };
+        return { success: false, error: getErrorMessage(err) };
       }
     });
 
@@ -868,7 +869,7 @@ export class WebServer extends EventEmitter {
         this.broadcast('case:linked', { name, path: expandedPath });
         return { success: true, case: { name, path: expandedPath } };
       } catch (err) {
-        return { success: false, error: (err as Error).message };
+        return { success: false, error: getErrorMessage(err) };
       }
     });
 
@@ -944,7 +945,7 @@ export class WebServer extends EventEmitter {
 
           this.broadcast('case:created', { name: caseName, path: casePath });
         } catch (err) {
-          return { success: false, error: `Failed to create case: ${(err as Error).message}` };
+          return { success: false, error: `Failed to create case: ${getErrorMessage(err)}` };
         }
       }
 
@@ -996,7 +997,7 @@ export class WebServer extends EventEmitter {
       } catch (err) {
         // Clean up session on error to prevent orphaned resources
         await this.cleanupSession(session.id);
-        return { success: false, error: (err as Error).message };
+        return { success: false, error: getErrorMessage(err) };
       }
     });
 
@@ -1026,7 +1027,7 @@ export class WebServer extends EventEmitter {
         writeFileSync(settingsPath, JSON.stringify(settings, null, 2));
         return { success: true };
       } catch (err) {
-        return { error: (err as Error).message };
+        return { error: getErrorMessage(err) };
       }
     });
 
@@ -1415,7 +1416,7 @@ export class WebServer extends EventEmitter {
         // Small pause between iterations
         await new Promise(r => setTimeout(r, 2000));
       } catch (err) {
-        addLog(`Error: ${(err as Error).message}`);
+        addLog(`Error: ${getErrorMessage(err)}`);
         this.broadcast('scheduled:updated', run);
 
         // Clean up the session on error too