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.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, test } from "bun:test"
 
-import { extractSessionIdFromOutput } from "./subagent-session-id"
+import { extractSessionIdFromMetadata, extractSessionIdFromOutput } from "./subagent-session-id"
 
 describe("extractSessionIdFromOutput", () => {
   test("extracts Session ID blocks from background output", () => {
@@ -77,3 +77,44 @@ debug log: session_id: ses_wrong_body_789`
     expect(result).toBe("ses_real_metadata_456")
   })
 })
+
+describe("extractSessionIdFromMetadata", () => {
+  test("extracts sessionId from tool metadata object", () => {
+    // given
+    const metadata = { sessionId: "ses_plugin_abc123" }
+
+    // when
+    const result = extractSessionIdFromMetadata(metadata)
+
+    // then
+    expect(result).toBe("ses_plugin_abc123")
+  })
+
+  test("returns undefined for metadata without sessionId", () => {
+    // given
+    const metadata = { title: "some task" }
+
+    // when
+    const result = extractSessionIdFromMetadata(metadata)
+
+    // then
+    expect(result).toBeUndefined()
+  })
+
+  test("returns undefined for non-object metadata", () => {
+    expect(extractSessionIdFromMetadata(null)).toBeUndefined()
+    expect(extractSessionIdFromMetadata(undefined)).toBeUndefined()
+    expect(extractSessionIdFromMetadata("string")).toBeUndefined()
+  })
+
+  test("rejects sessionId values that don't start with ses_", () => {
+    // given
+    const metadata = { sessionId: "not-a-session-id" }
+
+    // when
+    const result = extractSessionIdFromMetadata(metadata)
+
+    // then
+    expect(result).toBeUndefined()
+  })
+})

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]

src/hooks/atlas/tool-execute-after.ts

@@ -14,7 +14,7 @@ import { shouldPauseForFinalWaveApproval } from "./final-wave-approval-gate"
 import { HOOK_NAME } from "./hook-name"
 import { DIRECT_WORK_REMINDER } from "./system-reminder-templates"
 import { isSisyphusPath } from "./sisyphus-path"
-import { extractSessionIdFromOutput, validateSubagentSessionId } from "./subagent-session-id"
+import { extractSessionIdFromMetadata, extractSessionIdFromOutput, validateSubagentSessionId } from "./subagent-session-id"
 import {
   buildCompletionGate,
   buildFinalWaveApprovalReminder,
@@ -105,7 +105,9 @@ export function createToolExecuteAfterHandler(input: {
       return
     }
 
-    if (toolInput.tool !== "task") {
+    const metadataSessionId = extractSessionIdFromMetadata(toolOutput.metadata)
+    const isPluginToolWithSession = toolInput.tool !== "task" && !!metadataSessionId
+    if (toolInput.tool !== "task" && !isPluginToolWithSession) {
       return
     }
 
@@ -115,6 +117,7 @@ export function createToolExecuteAfterHandler(input: {
       pendingTaskRefs.delete(toolInput.callID)
     }
     const isBackgroundLaunch = outputStr.includes("Background task launched") || outputStr.includes("Background task continued")
+      || outputStr.includes("Background delegate launched")
     if (isBackgroundLaunch) {
       return
     }
@@ -125,7 +128,7 @@ export function createToolExecuteAfterHandler(input: {
       const verificationDirectory = worktreePath ? worktreePath : ctx.directory
       const gitStats = collectGitDiffStats(verificationDirectory)
       const fileChanges = formatFileChanges(gitStats)
-      const extractedSessionId = extractSessionIdFromOutput(toolOutput.output)
+      const extractedSessionId = metadataSessionId ?? extractSessionIdFromOutput(toolOutput.output)
 
       if (boulderState) {
         const progress = getPlanProgress(boulderState.active_plan)