fix(background-agent): release concurrency before prompt to unblock queued tasks

Previously, concurrency was released in finally block AFTER prompt completion.
This caused queued tasks to remain blocked while prompt hangs.

Now release happens BEFORE prompt, allowing next queued task to start immediately
when current task completes, regardless of prompt success/failure.

Also added early release on session creation error for proper cleanup.

src/features/background-agent/manager.test.ts

@@ -302,6 +302,74 @@ describe("BackgroundManager.getAllDescendantTasks", () => {
   })
 })
 
+describe("BackgroundManager.notifyParentSession - release ordering", () => {
+  test("should unblock queued task even when prompt hangs", async () => {
+    // #given - concurrency limit 1, task1 running, task2 waiting
+    const { ConcurrencyManager } = await import("./concurrency")
+    const concurrencyManager = new ConcurrencyManager({ defaultConcurrency: 1 })
+
+    await concurrencyManager.acquire("explore")
+
+    let task2Resolved = false
+    const task2Promise = concurrencyManager.acquire("explore").then(() => {
+      task2Resolved = true
+    })
+
+    await Promise.resolve()
+    expect(task2Resolved).toBe(false)
+
+    // #when - simulate notifyParentSession: release BEFORE prompt (fixed behavior)
+    let promptStarted = false
+    const simulateNotifyParentSession = async () => {
+      concurrencyManager.release("explore")
+
+      promptStarted = true
+      await new Promise(() => {})
+    }
+
+    simulateNotifyParentSession()
+
+    await Promise.resolve()
+    await Promise.resolve()
+
+    // #then - task2 should be unblocked even though prompt never completes
+    expect(promptStarted).toBe(true)
+    await task2Promise
+    expect(task2Resolved).toBe(true)
+  })
+
+  test("should keep queue blocked if release is after prompt (demonstrates the bug)", async () => {
+    // #given - same setup
+    const { ConcurrencyManager } = await import("./concurrency")
+    const concurrencyManager = new ConcurrencyManager({ defaultConcurrency: 1 })
+
+    await concurrencyManager.acquire("explore")
+
+    let task2Resolved = false
+    concurrencyManager.acquire("explore").then(() => {
+      task2Resolved = true
+    })
+
+    await Promise.resolve()
+    expect(task2Resolved).toBe(false)
+
+    // #when - simulate BUGGY behavior: release AFTER prompt (in finally)
+    const simulateBuggyNotifyParentSession = async () => {
+      try {
+        await new Promise((_, reject) => setTimeout(() => reject(new Error("timeout")), 50))
+      } finally {
+        concurrencyManager.release("explore")
+      }
+    }
+
+    await simulateBuggyNotifyParentSession().catch(() => {})
+
+    // #then - task2 resolves only after prompt completes (blocked during hang)
+    await Promise.resolve()
+    expect(task2Resolved).toBe(true)
+  })
+})
+
 describe("BackgroundManager.pruneStaleTasksAndNotifications", () => {
   let manager: MockBackgroundManager
 

src/features/background-agent/manager.ts

@@ -86,9 +86,13 @@ export class BackgroundManager {
         parentID: input.parentSessionID,
         title: `Background: ${input.description}`,
       },
+    }).catch((error) => {
+      this.concurrencyManager.release(model)
+      throw error
     })
 
     if (createResult.error) {
+      this.concurrencyManager.release(model)
       throw new Error(`Failed to create background session: ${createResult.error}`)
     }
 
@@ -345,6 +349,10 @@ export class BackgroundManager {
 
     const taskId = task.id
     setTimeout(async () => {
+      if (task.model) {
+        this.concurrencyManager.release(task.model)
+      }
+
       try {
         const messageDir = getMessageDir(task.parentSessionID)
         const prevMessage = messageDir ? findNearestMessageWithFields(messageDir) : null
@@ -367,10 +375,6 @@ export class BackgroundManager {
       } catch (error) {
         log("[background-agent] prompt failed:", String(error))
       } finally {
-        if (task.model) {
-          this.concurrencyManager.release(task.model)
-        }
-        // Always clean up both maps to prevent memory leaks
         this.clearNotificationsForTask(taskId)
         this.tasks.delete(taskId)
         log("[background-agent] Removed completed task from memory:", taskId)