refactor(agents): move spawn family into subagents/spawn (#121350)

* refactor(agents): move spawn family into subagents/spawn

* refactor(agents): keep spawn imports within lint budget

qa/scenarios/agents/subagent-fanout-synthesis.yaml

@@ -18,7 +18,7 @@ scenario:
     - docs/tools/subagents.md
     - docs/help/testing.md
   codeRefs:
-    - src/agents/subagent-spawn.ts
+    - src/agents/subagents/spawn/subagent-spawn.ts
     - src/agents/system-prompt.ts
     - extensions/qa-lab/src/suite.ts
   execution:

qa/scenarios/agents/subagent-forked-context.yaml

@@ -17,7 +17,7 @@ scenario:
     - docs/concepts/session-tool.md
   codeRefs:
     - src/agents/tools/sessions-spawn-tool.ts
-    - src/agents/subagent-spawn.ts
+    - src/agents/subagents/spawn/subagent-spawn.ts
   execution:
     kind: flow
     summary: Ask the agent to delegate work that depends on the current transcript and assert sessions_spawn carries context=fork.