feat(subagent): enrich subagent/start + subagent/end lifecycle events (observe-only)

A hooks bridge translating SubagentStart/SubagentStop needs to know WHICH kind of
subagent ran and WHAT it produced — Claude Code's hooks carry subagent_type and the
child's final message. Enrich the existing lifecycle emits to match, observe-only:

- agentType: an optional caller-supplied subagent-kind label (CC's subagent_type),
  added to SubagentStartRequest and carried VERBATIM onto both subagent/start
  (SubagentRunInfo) and subagent/end (SubagentRunEndInfo). The seam never interprets
  it. dsh-tool-subagent threads it from a new optional Config.agentType, so a
  deployment exposing multiple subagent kinds (one tool load per kind) labels each.
- lastAssistantMessage: the child's final output (SubagentResult.output), added to
  SubagentRunEndInfo on the settle path so an observer sees what the subagent
  produced without holding the run. Absent on the reject path (no result produced).

Strictly observe-only: both events stay plain emits (subagent/end fires from a
detached .then and awaits no listener). A control-flow subagent/end (awaited
waterfall returning a decision) would need the emit→waterfall reshape, awaiting
listeners before settling, and a provider resume capability — deferred to the
background/steering redesign (FIXME(subagent-continuation) anchors it). RFC:
implemented/feature/2026-06-30-subagent-observe-enrich.md.

packages/subagent/tool-subagent/README.md

@@ -11,6 +11,7 @@ This plugin binds to **exactly one** provider (`Config.provider`). The model see
 | `provider` (required) | The `ctx.subagents` provider name to start runs on (`spawn`, `fork`, `acp`, …). |
 | `toolName` | The model-facing tool name to register (default `subagent`). Set a distinct value per load when exposing multiple providers, e.g. `subagent` + `subagent_acp`. |
 | `agentOptions` | Default per-child `{ model?, systemPrompt? }` applied to every spawned child. |
+| `agentType` | Optional subagent-kind label (Claude Code's `subagent_type`) stamped on every run's `subagent/start`/`subagent/end` events, so an observer (a hooks bridge, a UI) can report or match on which kind ran. Set a distinct value per load when exposing multiple subagent kinds. |
 
 ## Lifecycle (synchronous collect)