feat: render subagent delegation events in chat UI

[rdwj]

Summary

Renders the three new `delta.subagent` SSE events emitted by fipsagents
0.22.0+ agents that use subagent-as-tool. Without this, the UI sees the
final assistant message but the delegation is invisible.

The wire format (verified against a deployed coordinator/specialist pair):

```json
{ "choices": [{ "delta": {
"subagent": {
"type": "invoked" | "completed" | "failed",
"agent_name": "...",
"span_id": "...",
...
}
}}] }
```

Span IDs correlate `completed` / `failed` events back to the originating
`invoked` event, so concurrent delegations render in parallel.

Behavior

• Invoked: a delegation card unfolds inline showing the target agent
  name, status ("delegating…"), and the task (truncated to 120 chars
  with a `
  Details

  ` toggle for the full string).
• Completed: card transitions to "done" state. Footer shows total
  tokens and cost. The subagent's `content` is not rendered in the
  card — it folds into the parent assistant message via normal `content`
  deltas so the answer reads naturally.
• Failed: card transitions to "error" state showing `error_type` and
  `error_message`.

What changed

File	LOC	Shape
`static/app.js`	+106	Three new helpers (`startSubagentCard`, `completeSubagentCard`, `failSubagentCard`) + a 12-line `if (delta.subagent)` dispatch in `handleDelta()`
`static/style.css`	+117	New `.subagent-delegation` ruleset mirroring `.tool-call` (per-state border colors, reusing the existing `pulse` keyframe)

Pure additive change. No edits to existing branches in the SSE dispatch,
the message accumulator, or the Go server (which is a verbatim SSE proxy).

Test plan

• `make build` succeeds (Go server embeds the new static files)
• Visual check against a deployed coordinator/specialist pair on
  OpenShift (target: `subagent-demo` namespace on `mcp-rhoai` cluster
  with a calculus-coordinator delegating to calculus-agent)
• Delegation card renders in three states (invoked / completed / failed)
• Concurrent delegations don't stomp each other (multiple span_ids)

Related

• agent-template PR #173 — subagent-as-tool v1
• agent-template #179 — v2 streaming nested deltas (will reuse the same card)