refactor(features/background-agent): remove dead code submodules

Delete 15 unused files in background-agent module:

- background-task-completer.ts

- format-duration.ts

- message-dir.ts

- parent-session-context-resolver.ts

- parent-session-notifier.ts (and its test file)

- result-handler-context.ts

- result-handler.ts

- session-output-validator.ts

- session-task-cleanup.ts

- session-todo-checker.ts

- spawner/background-session-creator.ts

- spawner/concurrency-key-from-launch-input.ts

- spawner/spawner-context.ts

- spawner/tmux-callback-invoker.ts

Update index.ts barrel and manager.ts/spawner.ts imports

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

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

src/features/background-agent/spawner/background-session-creator.ts

@@ -1,45 +0,0 @@
-import type { OpencodeClient } from "../constants"
-import type { ConcurrencyManager } from "../concurrency"
-import type { LaunchInput } from "../types"
-import { log } from "../../../shared"
-
-export async function createBackgroundSession(options: {
-  client: OpencodeClient
-  input: LaunchInput
-  parentDirectory: string
-  concurrencyManager: ConcurrencyManager
-  concurrencyKey: string
-}): Promise<string> {
-  const { client, input, parentDirectory, concurrencyManager, concurrencyKey } = options
-
-  const body = {
-    parentID: input.parentSessionID,
-    title: `Background: ${input.description}`,
-  }
-
-  const createResult = await client.session
-    .create({
-      body,
-      query: {
-        directory: parentDirectory,
-      },
-    })
-    .catch((error: unknown) => {
-      concurrencyManager.release(concurrencyKey)
-      throw error
-    })
-
-  if (createResult.error) {
-    concurrencyManager.release(concurrencyKey)
-    throw new Error(`Failed to create background session: ${createResult.error}`)
-  }
-
-  if (!createResult.data?.id) {
-    concurrencyManager.release(concurrencyKey)
-    throw new Error("Failed to create background session: API returned no session ID")
-  }
-
-  const sessionID = createResult.data.id
-  log("[background-agent] Background session created", { sessionID })
-  return sessionID
-}

src/features/background-agent/spawner/concurrency-key-from-launch-input.ts

@@ -1,7 +0,0 @@
-import type { LaunchInput } from "../types"
-
-export function getConcurrencyKeyFromLaunchInput(input: LaunchInput): string {
-  return input.model
-    ? `${input.model.providerID}/${input.model.modelID}`
-    : input.agent
-}

src/features/background-agent/spawner/spawner-context.ts

@@ -1,12 +0,0 @@
-import type { BackgroundTask } from "../types"
-import type { ConcurrencyManager } from "../concurrency"
-import type { OpencodeClient, OnSubagentSessionCreated } from "../constants"
-
-export interface SpawnerContext {
-  client: OpencodeClient
-  directory: string
-  concurrencyManager: ConcurrencyManager
-  tmuxEnabled: boolean
-  onSubagentSessionCreated?: OnSubagentSessionCreated
-  onTaskError: (task: BackgroundTask, error: Error) => void
-}

src/features/background-agent/spawner/tmux-callback-invoker.ts

@@ -1,40 +0,0 @@
-import { setTimeout } from "timers/promises"
-import type { OnSubagentSessionCreated } from "../constants"
-import { TMUX_CALLBACK_DELAY_MS } from "../constants"
-import { log } from "../../../shared"
-import { isInsideTmux } from "../../../shared/tmux"
-
-export async function maybeInvokeTmuxCallback(options: {
-  onSubagentSessionCreated?: OnSubagentSessionCreated
-  tmuxEnabled: boolean
-  sessionID: string
-  parentID: string
-  title: string
-}): Promise<void> {
-  const { onSubagentSessionCreated, tmuxEnabled, sessionID, parentID, title } = options
-
-  log("[background-agent] tmux callback check", {
-    hasCallback: !!onSubagentSessionCreated,
-    tmuxEnabled,
-    isInsideTmux: isInsideTmux(),
-    sessionID,
-    parentID,
-  })
-
-  if (!onSubagentSessionCreated || !tmuxEnabled || !isInsideTmux()) {
-    log("[background-agent] SKIP tmux callback - conditions not met")
-    return
-  }
-
-  log("[background-agent] Invoking tmux callback NOW", { sessionID })
-  await onSubagentSessionCreated({
-    sessionID,
-    parentID,
-    title,
-  }).catch((error: unknown) => {
-    log("[background-agent] Failed to spawn tmux pane:", error)
-  })
-
-  log("[background-agent] tmux callback completed, waiting")
-  await setTimeout(TMUX_CALLBACK_DELAY_MS)
-}