Fix review findings: lifecycle containment, configurable tool name, coverage, type catalog

Address four findings from the first Codex review round:

- Contain subagent/start|end listener throws (emitContainedStart/End): a
  thrown lifecycle listener could escape SubagentService.start() before the
  caller received the live run to dispose it (a leaked child), and a thrown
  subagent/end listener could surface as an unhandled rejection on the detached
  result-settle hook. Both emits now log-and-contain, mirroring the agent
  registry's agent/created|disposed containment.
- Make the model-facing tool name configurable (Config.toolName, default
  subagent). The docs say to load dsh-tool-subagent once per provider to expose
  multiple transports, but the hardcoded name made the second load throw a
  duplicate-tool-name error; a distinct toolName per load is now required and
  documented.
- Reach the per-file 100% coverage gate: tests for the subagent/end error
  branch, lifecycle-listener containment, every stopReasonError arm + the
  merge-extensible default, the multi-provider toolName path, agentOptions
  forwarding, and the direct-apply schema-bypass fallbacks.
- Document the seam vocabulary in docs/core-data-structures/subagent.md with
  verbatim type-equiv blocks + manifest entries, and link it from core.md (a
  brand-new core/seam type the doc-sync gate cannot detect on its own).

packages/support/subagent-mock/tests/subagent-mock.spec.ts

@@ -45,6 +45,12 @@ describe('dsh-subagent-mock', () => {
     await expect(run.result).resolves.toMatchObject({ structured: { answer: 42 } })
   })
 
+  it('defaults structured output to { reply } when outputSchema is requested but no structured value is configured', async () => {
+    const ctx = await mount({ reply: 'fallback reply' })
+    const run = ctx.subagents.start('mock', baseRequest({ outputSchema: { answer: { type: 'number' } } }))
+    await expect(run.result).resolves.toMatchObject({ structured: { reply: 'fallback reply' } })
+  })
+
   it('omits structured output when outputSchema capability is off', async () => {
     const ctx = await mount({ capabilities: { outputSchema: false } })
     // The service rejects an outputSchema request against a no-cap provider, so