feat: subagent-as-tool v2 — streaming nested deltas

[rdwj]

Summary

v1 of subagent-as-tool (#165, PR #173) ships with buffered subagent execution: the
parent waits for the subagent to finish and emits a single tool result. v2 streams
the subagent's events as nested SubagentDelta events on the parent's stream so
the UI can render delegation in real time.

The forward-compat plumbing already exists. PR #173 reserved a SubagentDelta
StreamEvent variant with no emitter; this issue is to wire it up end-to-end.

What lands

• SubagentTransport.execute_streaming() (or equivalent) returns an async
  iterator of child StreamEvents instead of a buffered SubagentResult
• The delegate_to_agent tool re-emits child events as
  SubagentDelta(agent_name, delta) on the parent's stream
• The parent's astep_stream consumer treats SubagentDelta as informational
  (does not feed it back to the model)
• Cost roll-up still happens at completion — _persist_cost_data continues to
  drain _subagent_token_usage once the subagent finishes
• UI rendering coordinated with fips-agents/ui-template (separate issue there)

Open design questions

1. Backpressure / dropping. If the parent's consumer is slow and the
   subagent is fast, do we buffer, drop, or apply backpressure? Design doc
   suggests a SubagentDeltaDropped warning event; pick one and document it.
2. Mid-stream subagent failure. When the subagent crashes after emitting
   partial deltas, what does the parent see? Likely a terminal SubagentFailed
   following the partial deltas, but spec the contract.
3. Nested delegation. A subagent that itself delegates — do we flatten the
   delta stream or wrap it twice? Flattening with agent_name chains is
   probably right; verify against trace structure.

Out of scope

• kagenti-driven discovery (separate issue)
• Remote-side depth enforcement (separate issue)

Dependencies

• Requires UI work in fips-agents/ui-template to render delegation visibly.

Related

• feat: subagent-as-tool — composable agent delegation #165 — v1 issue (closed by PR feat: subagent-as-tool v1 (#165) #173)
• Design doc: planning/subagent-tool-design.md ("Streaming roadmap" section)