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/tool-subagent/tests/tool-subagent.spec.ts

@@ -153,60 +153,6 @@ describe('dsh-tool-subagent', () => {
     expect(seen?.agentOptions).toEqual({ model: 'child-model', systemPrompt: 'be terse' })
   })
 
-  it('forwards a configured agentType into the start request (observed on the lifecycle events)', async () => {
-    let seen: { agentType?: string } | undefined
-    const starts: { agentType?: string }[] = []
-    const ctx = new Context()
-    await ctx.plugin(SystemPrompt)
-    await ctx.plugin(ToolRegistry)
-    await ctx.plugin(SubagentService)
-    ctx.on('subagent/start', info => void starts.push(info))
-    ctx.subagents.registerProvider({
-      name: 'typed',
-      capabilities: { outputSchema: false, depthLimit: false, toolFilter: false },
-      start: (request) => {
-        seen = request
-        return {
-          id: AgentId('typed-child'),
-          result: Promise.resolve({ output: [{ type: 'text', text: 'ok' }], stopReason: 'completed' as const }),
-          cancel() {},
-          dispose: async () => {},
-        }
-      },
-    })
-    await ctx.plugin(tool, { provider: 'typed', agentType: 'code-reviewer' })
-
-    await callSubagent(ctx, { description: 'd', prompt: 'p' })
-    // The config agentType reaches the request, and the service stamps it on the event.
-    expect(seen?.agentType).toBe('code-reviewer')
-    expect(starts[0]?.agentType).toBe('code-reviewer')
-  })
-
-  it('omits agentType from the request when none is configured', async () => {
-    let seen: { agentType?: string } | undefined
-    const ctx = new Context()
-    await ctx.plugin(SystemPrompt)
-    await ctx.plugin(ToolRegistry)
-    await ctx.plugin(SubagentService)
-    ctx.subagents.registerProvider({
-      name: 'untyped',
-      capabilities: { outputSchema: false, depthLimit: false, toolFilter: false },
-      start: (request) => {
-        seen = request
-        return {
-          id: AgentId('untyped-child'),
-          result: Promise.resolve({ output: [{ type: 'text', text: 'ok' }], stopReason: 'completed' as const }),
-          cancel() {},
-          dispose: async () => {},
-        }
-      },
-    })
-    await ctx.plugin(tool, { provider: 'untyped' })
-
-    await callSubagent(ctx, { description: 'd', prompt: 'p' })
-    expect(seen !== undefined && 'agentType' in seen).toBe(false)
-  })
-
   it('defaults toolName and omits agentOptions when apply() is called directly (schema bypass)', async () => {
     // `ctx.plugin` validates+defaults config first (toolName→'subagent', the
     // agentOptions object→{}), so the runtime `?? 'subagent'` fallback and the