Agent Teams: support preset selection in spawn_teammate

[Kiramuxa]

In DSH 0.2.1-rc.5, spawn_teammate does not expose preset selection:
teammates inherit the lead's composition. Allowing an independent preset would
let members of the same team use different tools and skills.
The underlying SubagentStartRequest already supports agentPreset.

Proposed change

Add an optional agentPreset parameter to the spawn_teammate schema and
SpawnTeammateRequest, then forward it as request.agentPreset when calling
subagents.startActivation.

Example of the proposed API:

spawn_teammate({
  name: "reviewer",
  description: "Code review",
  prompt: "Review the changes and report your findings",
  context: "fresh",
  agentPreset: "code-review"
});

Expected behavior

• An explicitly selected preset supplies the teammate's tools and skills before
  its first turn.
• The preset survives subsequent messages, automatic cold resume, and DSH restarts.
• Omitting the parameter preserves the current inheritance of the lead's composition.
• Empty or unknown preset IDs, and an independent preset combined with fork,
  are rejected before adding the teammate to the roster.

Cover these cases with regression tests and document the parameter in the
Agent Teams documentation.

Workarounds and limitations

• agentPresets.select() during agent/created. The intended preset applies
  before the first turn, but the next send_message after the live child is
  released restores the lead's preset. In rc.5, agent-preset/selected is recorded
  after subagent/descriptor, while restoration recognizes an independent
  selection only when it precedes the descriptor. No process restart is needed
  to reproduce this.
• Calling select() again during resume. This fails with agent-preset/locked
  (This session has already started), because the session has already run a turn.
• Calling mount() during resume. A local scenario with separate MCP servers
  and a follow-up passed, but the method's documented contract expects an
  unpublished Agent in a setup callback. During agent/created, the Agent has
  already been published. This workaround depends on lifecycle ordering and needs
  explicit restoration of the persisted selection. A full DSH restart was not
  tested for this workaround.
• A local tool/service patch. Forwarding agentPreset into startActivation
  passed a scenario with separate MCP servers, messaging, and automatic cold resume.
  However, this requires maintaining modified copies of the official modules
  across SDK updates.

These checks used rc.5, the actual Agent Teams runtime, and two local stdio MCP
servers. The proposed parameter would use the existing native preset-selection
mechanism at creation time, without late switching or custom restoration logic.