fix(atlas): restrict boulder continuation to sessions in boulder session_ids

Main session was unconditionally allowed through the boulder session guard,
causing continuation injection into sessions not part of the active boulder.
Now only sessions explicitly in boulder's session_ids (or background tasks)
receive boulder continuation, matching todo-continuation-enforcer behavior.

src/hooks/atlas/event-handler.ts

@@ -1,6 +1,6 @@
 import type { PluginInput } from "@opencode-ai/plugin"
 import { getPlanProgress, readBoulderState } from "../../features/boulder-state"
-import { getMainSessionID, subagentSessions } from "../../features/claude-code-session-state"
+import { subagentSessions } from "../../features/claude-code-session-state"
 import { log } from "../../shared/logger"
 import { HOOK_NAME } from "./hook-name"
 import { isAbortError } from "./is-abort-error"
@@ -43,13 +43,11 @@ export function createAtlasEventHandler(input: {
       const boulderState = readBoulderState(ctx.directory)
       const isBoulderSession = boulderState?.session_ids.includes(sessionID) ?? false
 
-      const mainSessionID = getMainSessionID()
-      const isMainSession = sessionID === mainSessionID
       const isBackgroundTaskSession = subagentSessions.has(sessionID)
 
-      // Allow continuation if: main session OR background task OR boulder session
-      if (mainSessionID && !isMainSession && !isBackgroundTaskSession && !isBoulderSession) {
-        log(`[${HOOK_NAME}] Skipped: not main, background task, or boulder session`, { sessionID })
+      // Allow continuation only if: session is in boulder's session_ids OR is a background task
+      if (!isBackgroundTaskSession && !isBoulderSession) {
+        log(`[${HOOK_NAME}] Skipped: not boulder or background task session`, { sessionID })
         return
       }