feat(background-task): make taskCleanupDelayMs configurable

TASK_CLEANUP_DELAY_MS is the delay between a task reaching a terminal
state (completed/cancelled/errored) and its removal from the in-memory
task store. It is currently a hard-coded 10 minute constant, which is
too short for long-running background workflows: users routinely hit
'task not found' on background_output lookups when they inspect
results more than ~10 minutes after completion.

taskTtlMs (landed in #2825) already exposes the non-terminal task TTL
on BackgroundTaskConfigSchema. This PR mirrors that pattern for the
terminal-state cleanup delay:

- Add taskCleanupDelayMs: z.number().min(60000).optional() to
  BackgroundTaskConfigSchema with JSDoc matching taskTtlMs's style.
- BackgroundManager.scheduleCompletionRemoval() reads
  this.config?.taskCleanupDelayMs ?? TASK_CLEANUP_DELAY_MS, preserving
  the existing 10 min default for unconfigured users.
- Regenerate assets/oh-my-opencode.schema.json.

Default: 600000 ms (10 min, unchanged from current hard-coded value).
Minimum: 60000 ms (1 min).

bun run typecheck: clean.
bun test src/features/background-agent: 411/411 pass.

assets/oh-my-opencode.schema.json

@@ -5687,6 +5687,10 @@
           "type": "number",
           "minimum": 10000
         },
+        "taskCleanupDelayMs": {
+          "type": "number",
+          "minimum": 60000
+        },
         "syncPollTimeoutMs": {
           "type": "number",
           "minimum": 60000

src/config/schema/background-task.ts

@@ -20,6 +20,8 @@ export const BackgroundTaskConfigSchema = z.object({
   taskTtlMs: z.number().min(300000).optional(),
   /** Timeout for tasks whose session has completely disappeared from the status registry (default: 60000 = 1 minute, minimum: 10000 = 10 seconds). When a session is gone (likely crashed), this shorter timeout is used instead of the normal stale timeout. */
   sessionGoneTimeoutMs: z.number().min(10000).optional(),
+  /** Delay before removing completed/cancelled/errored tasks from memory in milliseconds (default: 600000 = 10 minutes, minimum: 60000 = 1 minute). */
+  taskCleanupDelayMs: z.number().min(60000).optional(),
   syncPollTimeoutMs: z.number().min(60000).optional(),
   /** Maximum tool calls per subagent task before circuit breaker triggers (default: 200, minimum: 10). Prevents runaway loops from burning unlimited tokens. */
   maxToolCalls: z.number().int().min(10).optional(),

src/features/background-agent/manager.ts

@@ -1510,7 +1510,7 @@ export class BackgroundManager {
         SessionCategoryRegistry.remove(task.sessionID)
       }
       log("[background-agent] Removed completed task from memory:", taskId)
-    }, TASK_CLEANUP_DELAY_MS)
+    }, this.config?.taskCleanupDelayMs ?? TASK_CLEANUP_DELAY_MS)
 
     this.completionTimers.set(taskId, timer)
   }