fix(background-agent): release interrupted task slots during startup cleanup

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

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

@@ -1361,6 +1361,73 @@ describe("BackgroundManager.tryCompleteTask", () => {
     }
   })
 
+  test("should release task concurrencyKey when startTask throws after assigning it", async () => {
+    // given
+    const concurrencyKey = "anthropic/claude-opus-4-6"
+    const concurrencyManager = getConcurrencyManager(manager)
+
+    const task = createMockTask({
+      id: "task-process-key-concurrency",
+      sessionID: "session-process-key-concurrency",
+      parentSessionID: "parent-process-key-concurrency",
+      status: "pending",
+      agent: "explore",
+    })
+    const input = {
+      description: task.description,
+      prompt: task.prompt,
+      agent: task.agent,
+      parentSessionID: task.parentSessionID,
+      parentMessageID: task.parentMessageID,
+      model: { providerID: "anthropic", modelID: "claude-opus-4-6" },
+    }
+    getTaskMap(manager).set(task.id, task)
+    getQueuesByKey(manager).set(concurrencyKey, [{ task, input }])
+
+    ;(manager as unknown as { startTask: (item: { task: BackgroundTask; input: typeof input }) => Promise<void> }).startTask = async (item) => {
+      item.task.concurrencyKey = concurrencyKey
+      throw new Error("startTask failed after assigning concurrencyKey")
+    }
+
+    // when
+    await processKeyForTest(manager, concurrencyKey)
+
+    // then
+    expect(concurrencyManager.getCount(concurrencyKey)).toBe(0)
+    expect(task.concurrencyKey).toBeUndefined()
+  })
+
+  test("should release queue slot when queued task is already interrupt", async () => {
+    // given
+    const concurrencyKey = "anthropic/claude-opus-4-6"
+    const concurrencyManager = getConcurrencyManager(manager)
+
+    const task = createMockTask({
+      id: "task-process-key-interrupt",
+      sessionID: "session-process-key-interrupt",
+      parentSessionID: "parent-process-key-interrupt",
+      status: "interrupt",
+      agent: "explore",
+    })
+    const input = {
+      description: task.description,
+      prompt: task.prompt,
+      agent: task.agent,
+      parentSessionID: task.parentSessionID,
+      parentMessageID: task.parentMessageID,
+      model: { providerID: "anthropic", modelID: "claude-opus-4-6" },
+    }
+    getTaskMap(manager).set(task.id, task)
+    getQueuesByKey(manager).set(concurrencyKey, [{ task, input }])
+
+    // when
+    await processKeyForTest(manager, concurrencyKey)
+
+    // then
+    expect(concurrencyManager.getCount(concurrencyKey)).toBe(0)
+    expect(getQueuesByKey(manager).get(concurrencyKey)).toEqual([])
+  })
+
   test("should avoid overlapping promptAsync calls when tasks complete concurrently", async () => {
     // given
     type PromptAsyncBody = Record<string, unknown> & { noReply?: boolean }
@@ -3216,7 +3283,7 @@ describe("BackgroundManager.handleEvent - session.error", () => {
       concurrencyKey,
       fallbackChain: [
         { providers: ["anthropic"], model: "claude-opus-4-6", variant: "max" },
-        { providers: ["anthropic"], model: "claude-opus-4-5" },
+        { providers: ["anthropic"], model: "claude-opus-4-5", variant: "max" },
       ],
     })
 
@@ -3298,21 +3365,23 @@ describe("BackgroundManager.handleEvent - session.error", () => {
     })
 
     //#when
+    const messageInfo = {
+      id: "msg_errored",
+      sessionID,
+      role: "assistant",
+      error: {
+        name: "UnknownError",
+        data: {
+          message:
+            "Bad Gateway: {\"error\":{\"message\":\"unknown provider for model claude-opus-4-6-thinking\"}}",
+        },
+      },
+    }
+
     manager.handleEvent({
       type: "message.updated",
       properties: {
-        info: {
-          id: "msg_errored",
-          sessionID,
-          role: "assistant",
-          error: {
-            name: "UnknownError",
-            data: {
-              message:
-                "Bad Gateway: {\"error\":{\"message\":\"unknown provider for model claude-opus-4-6-thinking\"}}",
-            },
-          },
-        },
+        info: messageInfo,
       },
     })