feat(atlas): support plugin tools with metadata.sessionId in orchestration hook

The tool-execute-after hook only processed the built-in 'task' tool.
Plugin tools (e.g. custom-agent-bridge delegate) that set
context.metadata({ metadata: { sessionId } }) were ignored by atlas
orchestration (boulder tracking, verification reminders).

- Add extractSessionIdFromMetadata() to read sessionId from tool
  metadata, preferred over text-based extraction
- Extend tool-execute-after guard to also process any tool that has
  metadata.sessionId set (not just tool === 'task')
- Recognize 'Background delegate launched' as a background launch
- Add tests for metadata-based session ID extraction

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

@@ -3,6 +3,16 @@ import { log } from "../../shared/logger"
 import { isSessionInBoulderLineage } from "./boulder-session-lineage"
 import { HOOK_NAME } from "./hook-name"
 
+export function extractSessionIdFromMetadata(metadata: unknown): string | undefined {
+  if (metadata && typeof metadata === "object" && "sessionId" in metadata) {
+    const value = (metadata as Record<string, unknown>).sessionId
+    if (typeof value === "string" && value.startsWith("ses_")) {
+      return value
+    }
+  }
+  return undefined
+}
+
 export function extractSessionIdFromOutput(output: string): string | undefined {
   const taskMetadataBlocks = [...output.matchAll(/<task_metadata>([\s\S]*?)<\/task_metadata>/gi)]
   const lastTaskMetadataBlock = taskMetadataBlocks.at(-1)?.[1]