[EPIC] Subagent orchestration

[ypldan]

Goal

Make QuickApps a deliberate multi-agent orchestrator rather than an agent that happens to call other agents as tools. Today the agent cannot tell a stateless tool from a subagent with shared history, cannot hold more than one conversation thread with the same subagent, and cannot delegate scoped work to a throwaway worker. These are three facets of one capability and should be planned together.

Scope — child issues

• Make agent aware of subagents #88
• Allow agent to create multiple separate conversations with the same agent #89
• Anonymous subagents #181

Out of scope

• Propagating subagent stages to the main agent (#87) — postponed until DIAL Chat supports nested stages; stays a standalone issue.
• Exposing QuickApps itself as a tool for other agents (#244 — opposite direction).

Dependencies and sequencing

• #88 first — it defines the conversation-state semantics the rest build on; PR feat: agent awareness of subagents conversation mode #403 is in flight.
• #89 extends Make agent aware of subagents #88's semantics with named threads; #181 is largely independent but should reuse the same conversation model.

Additional information

Related: #244 (the inverse integration), docs/agent.md orchestrator loop.