fix(headless): compose the one-shot agent from the preset roster again

The agent-default-model refactor moved the headless runner off the
api-proxy create path onto a direct agents.create, which silently dropped
the preset composition the stack had put on every dsh run session: the
header lost its agentPreset and the agent ran on the host composition
alone. Resolve the default preset up front (optional service — a roster-
less composition keeps the pre-preset behavior), record it on the header,
and mount it in setup where a broken composition still fails the run.

packages/bundle/headless/tsconfig.json

@@ -23,11 +23,14 @@
     {
       "path": "../../core/agent-default-model"
     },
+    {
+      "path": "../../core/session"
+    },
     {
       "path": "../../llm/llm"
     },
     {
-      "path": "../../core/session"
+      "path": "../../preset/agent-presets"
     },
     {
       "path": "../../support/invariants"