Fix ACP error handling to use event-driven pattern

Change error handling in sendPrompt from throwing exceptions to emitting
session update events. This ensures errors appear inline in the chat
instead of triggering IPC failures and toast notifications.

- Emit error as agent_message_chunk instead of throwing
- Return 'error' stopReason for caller awareness
- Add parseAcpError helper for user-friendly messages

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

src/main/conductor/Conductor.ts

@@ -409,6 +409,40 @@ export class Conductor {
       console.log(`[Conductor] Prompt completed with stopReason: ${result.stopReason}`)
 
       return result.stopReason
+    } catch (error) {
+      console.error(`[Conductor] ACP error for session ${sessionId}:`, error)
+
+      // Parse ACP error to user-friendly message
+      const message = this.parseAcpError(error)
+
+      // DESIGN DECISION: Event-driven error handling instead of throwing
+      //
+      // Why not throw?
+      // - Throwing causes IPC handler to fail, frontend receives an error response
+      // - This typically triggers a toast/dialog, interrupting user flow
+      //
+      // Why emit an event?
+      // - Error appears inline in the chat as a message, keeping context visible
+      // - User can read the error and continue the conversation naturally
+      // - Consistent with how agent responses are already delivered (via events)
+      //
+      // This pattern is similar to how OpenWork handles task errors:
+      // errors are forwarded to the renderer as updates, not thrown as exceptions.
+      if (this.events.onSessionUpdate) {
+        this.events.onSessionUpdate({
+          sessionId: agentSessionId,
+          update: {
+            sessionUpdate: 'agent_message_chunk',
+            content: {
+              type: 'text',
+              text: `\n\n**Error:** ${message}\n`
+            }
+          }
+        } as SessionNotification)
+      }
+
+      // Return 'error' as stopReason - IPC succeeds, caller knows it was an error
+      return 'error'
     } finally {
       // Always remove from processing when done (success or error)
       this.processingSessions.delete(sessionId)
@@ -641,6 +675,32 @@ export class Conductor {
     return this.processingSessions.has(sessionId)
   }
 
+  /**
+   * Parse ACP error and return user-friendly message
+   */
+  private parseAcpError(error: unknown): string {
+    const errorStr = String(error)
+
+    // MCP server missing environment variables
+    if (errorStr.includes('Missing environment variables')) {
+      const match = errorStr.match(/Missing environment variables: ([A-Z_]+)/)
+      return `MCP server requires environment variable: ${match?.[1] || 'unknown'}`
+    }
+
+    // File too large to read
+    if (errorStr.includes('MaxFileReadTokenExceededError')) {
+      return 'File is too large to read. Try reading smaller portions.'
+    }
+
+    // MCP configuration invalid
+    if (errorStr.includes('mcp-config-invalid')) {
+      return 'MCP server configuration is invalid. Check your settings.'
+    }
+
+    // Generic fallback
+    return 'Agent encountered an error. Please try again.'
+  }
+
   /**
    * Find Multica session ID by ACP agent session ID
    */