feat(athena-junior): add athena_council bulk launch tool

- Create src/tools/athena-council/ with council-launcher, tool factory, types
- Launch all council members in parallel via BackgroundManager with writeOutputToFile: true
- Wire tool into registry with backgroundManager + councilConfig dependencies
- Grant athena-junior athena_council permission, remove task permission
- Update non-interactive prompt to use athena_council instead of task tool
- Add explicit stop-after-result instruction to prevent post-output text
- 23 tests (14 tool + 9 launcher) using real temp files, no mock.module

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

@@ -0,0 +1,52 @@
+import type { BackgroundManager } from "../../features/background-agent"
+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"
+
+export interface CouncilLaunchContext {
+  parentSessionID: string
+  parentMessageID: string
+  parentAgent?: string
+}
+
+interface LaunchOutcome {
+  member: CouncilMemberConfig
+  task: BackgroundTask
+}
+
+/**
+ * Launches a single council member as a background task.
+ * The agent key follows the "Council: <name>" pattern used by council-member-agents.ts.
+ */
+export async function launchCouncilMember(
+  member: CouncilMemberConfig,
+  prompt: string,
+  manager: BackgroundManager,
+  context: CouncilLaunchContext,
+): Promise<LaunchOutcome> {
+  const parsed = parseModelString(member.model)
+  if (!parsed) {
+    throw new Error(`Invalid model format: "${member.model}" (expected "provider/model-id")`)
+  }
+
+  const agentKey = `${COUNCIL_MEMBER_KEY_PREFIX}${member.name}`
+  const memberName = member.name ?? member.model
+
+  const task = await manager.launch({
+    description: `Council member: ${memberName}`,
+    prompt,
+    agent: agentKey,
+    parentSessionID: context.parentSessionID,
+    parentMessageID: context.parentMessageID,
+    parentAgent: context.parentAgent,
+    writeOutputToFile: true,
+    model: {
+      providerID: parsed.providerID,
+      modelID: parsed.modelID,
+      ...(member.variant ? { variant: member.variant } : {}),
+    },
+  })
+
+  return { member, task }
+}