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()
+  })
+})