fix(atlas): harden task session reuse

src/hooks/atlas/subagent-session-id.ts

@@ -1,3 +1,8 @@
+import type { PluginInput } from "@opencode-ai/plugin"
+import { log } from "../../shared/logger"
+import { isSessionInBoulderLineage } from "./boulder-session-lineage"
+import { HOOK_NAME } from "./hook-name"
+
 export function extractSessionIdFromOutput(output: string): string | undefined {
   const taskMetadataBlocks = [...output.matchAll(/<task_metadata>([\s\S]*?)<\/task_metadata>/gi)]
   const lastTaskMetadataBlock = taskMetadataBlocks.at(-1)?.[1]
@@ -11,3 +16,29 @@ export function extractSessionIdFromOutput(output: string): string | undefined {
   const explicitSessionMatches = [...output.matchAll(/Session ID:\s*(ses_[a-zA-Z0-9_]+)/g)]
   return explicitSessionMatches.at(-1)?.[1]
 }
+
+export async function validateSubagentSessionId(input: {
+  client: PluginInput["client"]
+  sessionID?: string
+  lineageSessionIDs: string[]
+}): Promise<string | undefined> {
+  if (!input.sessionID || input.lineageSessionIDs.length === 0) {
+    return undefined
+  }
+
+  const belongsToLineage = await isSessionInBoulderLineage({
+    client: input.client,
+    sessionID: input.sessionID,
+    boulderSessionIDs: input.lineageSessionIDs,
+  })
+
+  if (!belongsToLineage) {
+    log(`[${HOOK_NAME}] Ignoring extracted session id outside active lineage`, {
+      sessionID: input.sessionID,
+      lineageSessionIDs: input.lineageSessionIDs,
+    })
+    return undefined
+  }
+
+  return input.sessionID
+}