fix(athena): per-task TTL for council members instead of blanket exemption

Add optional ttl field to BackgroundTask/LaunchInput. Council launcher
passes COUNCIL_MEMBER_TTL_MS (2 hours) per member. Task poller uses
task.ttl ?? TASK_TTL_MS, replacing the blanket TTL_EXEMPT_AGENTS bypass
for council members with a bounded timeout.

Co-authored-by: Vacbo <2445>

src/agents/athena/constants.ts

@@ -4,4 +4,5 @@ export const COUNCIL_DEFAULTS = {
   STUCK_THRESHOLD_SECONDS: 120,
   MEMBER_MAX_RUNNING_SECONDS: 1800,
   ARCHIVE_ID_BYTES: 8,
+  COUNCIL_MEMBER_TTL_MS: 2 * 60 * 60 * 1000,
 } as const

src/tools/athena-council/council-launcher.ts

@@ -3,6 +3,7 @@ import type { BackgroundTask } from "../../features/background-agent/types"
 import type { CouncilMemberConfig } from "../../config/schema/athena"
 import { parseModelString } from "../delegate-task/model-string-parser"
 import { COUNCIL_MEMBER_KEY_PREFIX } from "../../agents/builtin-agents/council-member-agents"
+import { COUNCIL_DEFAULTS } from "../../agents/athena"
 
 export interface CouncilLaunchContext {
   parentSessionID: string
@@ -46,6 +47,7 @@ export async function launchCouncilMember(
       modelID: parsed.modelID,
       ...(member.variant ? { variant: member.variant } : {}),
     },
+      ttl: COUNCIL_DEFAULTS.COUNCIL_MEMBER_TTL_MS,
   })
 
   return { member, task }