quill: add PRD draft for parallel subagent delegation

wizard-work/quill/prd-parallel-subagents.md

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+# PRD: Parallel Subagent Delegation in Hermes Agent
+
+## Problem
+Currently, delegating multiple tasks via `delegate_task` requires separate calls, limiting throughput for independent workstreams.
+
+## Proposed Solution
+Extend `delegate_task` to accept up to 10 tasks in a single call, running them in parallel and returning an array of summaries. Each subagent receives isolated context and terminal session; parent orchestrates results.
+
+## Success Criteria
+- Single `delegate_task` call with 10 tasks returns 10 summaries within 60s.
+- Failure of one subagent does not block others; partial results are returned.
+- No change to existing single-task behavior.
+
+## Scope
+In scope: parallel dispatch, result aggregation, error isolation. Out of scope: persistent subagent pools, inter-subagent messaging.
+
+## Open Questions
+- Should max concurrency be configurable beyond 10?
+- Should subagents inherit the parent's model or allow per-task model pinning?