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/subagent/tool-subagent/src/index.ts

@@ -35,6 +35,14 @@ export const inject = ['tools', 'subagents']
 export interface Config {
   /** The `ctx.subagents` provider name to start runs on (e.g. `spawn`, `acp`). */
   provider: string
+  /**
+   * The model-facing tool name to register (default `subagent`). To expose more
+   * than one transport, load this plugin once per provider — each load MUST set
+   * a distinct `toolName` (the tool registry rejects a duplicate name), e.g.
+   * `{ provider: 'spawn', toolName: 'subagent' }` and
+   * `{ provider: 'acp', toolName: 'subagent_acp' }`.
+   */
+  toolName?: string
   /**
    * Default per-child agent options (model, system prompt) applied to every
    * spawned child. Omitted fields fall back to the child loop's own defaults.
@@ -44,6 +52,7 @@ export interface Config {
 
 export const Config: z<Config> = z.object({
   provider: z.string().required(),
+  toolName: z.string().default('subagent'),
   agentOptions: z.object({
     model: z.string(),
     systemPrompt: z.string(),
@@ -85,7 +94,7 @@ function stopReasonError(result: SubagentResult): string | undefined {
 
 export function apply(ctx: Context, config: Config): void {
   ctx.tools.register(defineTool({
-    name: 'subagent',
+    name: config.toolName ?? 'subagent',
     description:
       'Delegate a self-contained task to a subagent (a separate agent that works in its own context) '
       + 'and return its final result. Use this to offload focused, independent work — research, a scoped '