refactor: Enhance session management and error handling in AgentService and related components

- Improved session handling by implementing ensureSession to load sessions from disk if not in memory, reducing "session not found" errors.
- Enhanced error messages for non-existent sessions, providing clearer diagnostics.
- Updated CodexProvider and OpencodeProvider to improve error handling and messaging.
- Refactored various routes to use async/await for better readability and error handling.
- Added event emission for merge and stash operations in the MergeService and StashService.
- Cleaned up error messages in AgentExecutor to remove redundant prefixes and ANSI codes for better clarity.

apps/server/src/routes/agent/routes/history.ts

@@ -16,7 +16,7 @@ export function createHistoryHandler(agentService: AgentService) {
         return;
       }
 
-      const result = agentService.getHistory(sessionId);
+      const result = await agentService.getHistory(sessionId);
       res.json(result);
     } catch (error) {
       logError(error, 'Get history failed');

apps/server/src/routes/agent/routes/queue-list.ts

@@ -19,7 +19,7 @@ export function createQueueListHandler(agentService: AgentService) {
         return;
       }
 
-      const result = agentService.getQueue(sessionId);
+      const result = await agentService.getQueue(sessionId);
       res.json(result);
     } catch (error) {
       logError(error, 'List queue failed');

apps/server/src/routes/agent/routes/send.ts

@@ -53,7 +53,15 @@ export function createSendHandler(agentService: AgentService) {
           thinkingLevel,
         })
         .catch((error) => {
-          logger.error('Background error in sendMessage():', error);
+          const errorMsg = (error as Error).message || 'Unknown error';
+          logger.error(`Background error in sendMessage() for session ${sessionId}:`, errorMsg);
+
+          // Emit error via WebSocket so the UI is notified even though
+          // the HTTP response already returned 200. This is critical for
+          // session-not-found errors where sendMessage() throws before it
+          // can emit its own error event (no in-memory session to emit from).
+          agentService.emitSessionError(sessionId, errorMsg);
+
           logError(error, 'Send message failed (background)');
         });
 

apps/server/src/routes/worktree/index.ts

@@ -80,7 +80,7 @@ export function createWorktreeRoutes(
     '/merge',
     validatePathParams('projectPath'),
     requireValidProject,
-    createMergeHandler()
+    createMergeHandler(events)
   );
   router.post(
     '/create',
@@ -126,7 +126,12 @@ export function createWorktreeRoutes(
     requireValidWorktree,
     createListBranchesHandler()
   );
-  router.post('/switch-branch', requireValidWorktree, createSwitchBranchHandler(events));
+  router.post(
+    '/switch-branch',
+    validatePathParams('worktreePath'),
+    requireValidWorktree,
+    createSwitchBranchHandler(events)
+  );
   router.post('/open-in-editor', validatePathParams('worktreePath'), createOpenInEditorHandler());
   router.post(
     '/open-in-terminal',
@@ -230,7 +235,7 @@ export function createWorktreeRoutes(
     '/stash-apply',
     validatePathParams('worktreePath'),
     requireGitRepoOnly,
-    createStashApplyHandler()
+    createStashApplyHandler(events)
   );
   router.post(
     '/stash-drop',

apps/server/src/routes/worktree/routes/merge.ts

@@ -9,9 +9,10 @@
 
 import type { Request, Response } from 'express';
 import { getErrorMessage, logError } from '../common.js';
+import type { EventEmitter } from '../../../lib/events.js';
 import { performMerge } from '../../../services/merge-service.js';
 
-export function createMergeHandler() {
+export function createMergeHandler(events: EventEmitter) {
   return async (req: Request, res: Response): Promise<void> => {
     try {
       const { projectPath, branchName, worktreePath, targetBranch, options } = req.body as {
@@ -34,7 +35,14 @@ export function createMergeHandler() {
       const mergeTo = targetBranch || 'main';
 
       // Delegate all merge logic to the service
-      const result = await performMerge(projectPath, branchName, worktreePath, mergeTo, options);
+      const result = await performMerge(
+        projectPath,
+        branchName,
+        worktreePath,
+        mergeTo,
+        options,
+        events
+      );
 
       if (!result.success) {
         if (result.hasConflicts) {

apps/server/src/routes/worktree/routes/stash-apply.ts

@@ -11,10 +11,11 @@
  */
 
 import type { Request, Response } from 'express';
+import type { EventEmitter } from '../../../lib/events.js';
 import { getErrorMessage, logError } from '../common.js';
 import { applyOrPop } from '../../../services/stash-service.js';
 
-export function createStashApplyHandler() {
+export function createStashApplyHandler(events: EventEmitter) {
   return async (req: Request, res: Response): Promise<void> => {
     try {
       const { worktreePath, stashIndex, pop } = req.body as {
@@ -50,7 +51,7 @@ export function createStashApplyHandler() {
       }
 
       // Delegate all stash apply/pop logic to the service
-      const result = await applyOrPop(worktreePath, idx, { pop });
+      const result = await applyOrPop(worktreePath, idx, { pop }, events);
 
       if (!result.success) {
         logError(new Error(result.error ?? 'Stash apply failed'), 'Stash apply failed');