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/package.json

@@ -38,6 +38,7 @@
   "peerDependencies": {
     "@deepseek-ai/dsh-agent": "^0.0.1",
     "@deepseek-ai/dsh-agent-default-model": "^0.0.1",
+    "@deepseek-ai/dsh-agent-presets": "^0.0.1",
     "@deepseek-ai/dsh-invariants": "^0.0.1",
     "@deepseek-ai/dsh-llm": "^0.0.1",
     "@deepseek-ai/dsh-session": "^0.0.1",
@@ -47,6 +48,7 @@
     "@cordisjs/plugin-loader": "workspace:^",
     "@deepseek-ai/dsh-agent": "workspace:^",
     "@deepseek-ai/dsh-agent-default-model": "workspace:^",
+    "@deepseek-ai/dsh-agent-presets": "workspace:^",
     "@deepseek-ai/dsh-invariants": "workspace:^",
     "@deepseek-ai/dsh-llm": "workspace:^",
     "@deepseek-ai/dsh-session": "workspace:^",