🤖 fix: address remaining review feedback

- Fix trunk branch detection for worktree/SSH runtimes: detect
  default trunk branch from parent project for non-local runtimes
- Apply preset systemPrompt via additionalSystemInstructions in
  startTask and rehydrateTasks continuation messages

Note: restart-safe foreground task waiting (parentToolCallId plumbing
to inject tool output in parent) requires significant additional
infrastructure and is deferred. The parentToolCallId is stored for
future use but not actively used for restart recovery yet.

Change-Id: I9d5a3d18f373564179e7a405a700b31e9a6739db
Signed-off-by: Thomas Kosiewski <tk@coder.com>

Author: ThomasK33

src/node/services/taskService.ts

@@ -23,6 +23,7 @@ import {
   type MuxFrontendMetadata,
 } from "@/common/types/message";
 import type { FrontendWorkspaceMetadata } from "@/common/types/workspace";
+import { detectDefaultTrunkBranch, listLocalBranches } from "@/node/git";
 import * as crypto from "crypto";
 
 export interface CreateTaskOptions {
@@ -163,12 +164,27 @@ export class TaskService extends EventEmitter {
     // For local workspaces, this has no change in behavior
     const parentRuntime = parentMetadata.runtimeConfig;
 
+    // Detect trunk branch for worktree/SSH runtimes
+    // Local runtime doesn't need a trunk branch
+    let trunkBranch = "";
+    const isLocalRuntime =
+      parentRuntime.type === "local" &&
+      !("srcBaseDir" in parentRuntime && parentRuntime.srcBaseDir);
+    if (!isLocalRuntime) {
+      try {
+        const branches = await listLocalBranches(parentInfo.projectPath);
+        trunkBranch = (await detectDefaultTrunkBranch(parentInfo.projectPath, branches)) ?? "main";
+      } catch (error) {
+        log.warn(`Failed to detect trunk branch for agent task, using 'main':`, error);
+        trunkBranch = "main";
+      }
+    }
+
     // Create workspace via WorkspaceService
-    // Using empty trunk branch because agent workspaces don't need git isolation
     const createResult = await this.workspaceService.create(
       parentInfo.projectPath,
       workspaceName,
-      "", // No trunk branch - agent tasks share parent's working directory context
+      trunkBranch,
       workspaceTitle,
       parentRuntime // Inherit runtime from parent workspace
     );
@@ -247,8 +263,8 @@ export class TaskService extends EventEmitter {
         model: "anthropic:claude-sonnet-4-20250514", // Default model for agents
         thinkingLevel: "medium",
         toolPolicy: preset.toolPolicy,
+        additionalSystemInstructions: preset.systemPrompt,
         mode: "exec", // Agent tasks are always in exec mode
-        // System prompt override happens in AIService based on agentType
       });
 
       if (!result.success) {
@@ -508,13 +524,15 @@ export class TaskService extends EventEmitter {
       } else if (taskState.taskStatus === "running") {
         // Send continuation message for running tasks
         try {
+          const preset = getAgentPreset(taskState.agentType);
           const result = await this.workspaceService.sendMessage(
             workspaceId,
             "Mux was restarted. Please continue your task and call agent_report when done.",
             {
               model: "anthropic:claude-sonnet-4-20250514",
               thinkingLevel: "medium",
-              toolPolicy: getAgentPreset(taskState.agentType).toolPolicy,
+              toolPolicy: preset.toolPolicy,
+              additionalSystemInstructions: preset.systemPrompt,
               mode: "exec",
             }
           );