refactor(subagent): drop the agentType lifecycle field

Address review: `agentType` was a Claude-Code concept (`subagent_type`) that
does not fit our own subagent seam — nothing in the harness interprets it, and
its only consumer was the CC-dialect hook bridge. Rather than let a foreign
concept sit on the core seam, remove it:

- `SubagentStartRequest`, `SubagentRunInfo`, `SubagentRunEndInfo`: drop the
  `agentType` field; the `subagent/start`/`subagent/end` payloads now carry
  `provider`/`id` (+ end `stopReason`/`lastAssistantMessage`) only.
- `dsh-tool-subagent`: drop `Config.agentType` and its request plumbing.
- Tests: keep the lastAssistantMessage / clone-containment / reject-path
  coverage (rewritten to not assert agentType); delete the two tool-subagent
  tests that only exercised agentType forwarding (dead behavior).
- Docs: retitle + rewrite the subagent-observe-enrich RFC to the one shipped
  enrichment (lastAssistantMessage), with a note on why agentType was dropped;
  update rfc/README index title, both subagent READMEs, and the
  core-data-structures/subagent.md type-equiv block + prose; regenerate catalog.

The CC bridge (PR-F) will feed Claude Code's own default matcher value
"general-purpose" for its SubagentStart/Stop agent_type matcher instead.

packages/subagent/subagent/src/index.ts

@@ -20,9 +20,9 @@
  * semantics are deferred to a future redesign that unifies long-running-tool
  * handling across subagents and bash.
  *
- * The `subagent/start` / `subagent/end` lifecycle events carry an enriched but
- * OBSERVE-ONLY payload (`agentType`, and on end `lastAssistantMessage`) — see
- * `docs/rfc/implemented/feature/2026-06-30-subagent-observe-enrich.md`.
+ * The `subagent/start` / `subagent/end` lifecycle events carry an OBSERVE-ONLY
+ * payload; `subagent/end` additionally carries the child's `lastAssistantMessage`
+ * — see `docs/rfc/implemented/feature/2026-06-30-subagent-observe-enrich.md`.
  * FIXME(subagent-continuation): a control-flow `subagent/end` (an awaited
  * waterfall returning a stop/continue decision, like the other interception
  * seams) would require reshaping this emit into a waterfall, awaiting listeners
@@ -82,13 +82,6 @@ export interface SubagentRunInfo {
   provider: string
   /** The child agent's id. */
   id: AgentId
-  /**
-   * The caller's subagent-kind label, carried verbatim from
-   * {@link SubagentStartRequest.agentType} (Claude Code's `subagent_type`).
-   * Absent when the caller did not supply one. An observer (a hooks bridge,
-   * a UI) reports or matches on it; the seam never interprets it.
-   */
-  agentType?: string
 }
 
 /** Outcome detail for a settled subagent run (the `subagent/end` payload). */
@@ -97,8 +90,6 @@ export interface SubagentRunEndInfo {
   provider: string
   /** The child agent's id. */
   id: AgentId
-  /** The caller's subagent-kind label (see {@link SubagentRunInfo.agentType}). */
-  agentType?: string
   /** The terminal stop reason. */
   stopReason: SubagentResult['stopReason']
   /**
@@ -187,10 +178,7 @@ export class SubagentService extends Service {
     // acceptable. `ctx.emit` halts the dispatch on the first throw, so a single
     // surrounding try/catch is not enough — each listener is invoked and
     // contained individually.
-    // Carry the caller's subagent-kind label verbatim onto both lifecycle events
-    // (absent when not supplied — the spread omits the key for exactOptionalPropertyTypes).
-    const agentType = request.agentType !== undefined ? { agentType: request.agentType } : {}
-    this.emitLifecycle('subagent/start', { provider: name, id: run.id, ...agentType })
+    this.emitLifecycle('subagent/start', { provider: name, id: run.id })
     // Emit `subagent/end` when the run settles. The result promise does not
     // reject on a child-level failure (it resolves with stopReason 'error'),
     // so a rejection here is an infrastructure fault — surface its stop reason
@@ -220,9 +208,9 @@ export class SubagentService extends Service {
         } catch (error: unknown) {
           this.ctx.logger.warn(`subagent: could not clone ${name} output for subagent/end: ${String(error)}`)
         }
-        this.emitLifecycle('subagent/end', { provider: name, id: run.id, ...agentType, stopReason: result.stopReason, ...lastAssistantMessage !== undefined ? { lastAssistantMessage } : {} })
+        this.emitLifecycle('subagent/end', { provider: name, id: run.id, stopReason: result.stopReason, ...lastAssistantMessage !== undefined ? { lastAssistantMessage } : {} })
       },
-      () => { this.emitLifecycle('subagent/end', { provider: name, id: run.id, ...agentType, stopReason: 'error' }) },
+      () => { this.emitLifecycle('subagent/end', { provider: name, id: run.id, stopReason: 'error' }) },
     )
     return run
   }