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).

docs/core-data-structures/core.md

@@ -20,6 +20,7 @@ Everything else is documented on a **sub-page**, not here. The rule that draws t
 | [persistence.md](persistence.md) | the durability seam: `SessionPersistence`, JSONL + SQLite backends, `session/flush`, crash recovery, `SessionHeader` |
 | [tools.md](tools.md) | `ToolDefinition` full fields, the schema DSL, `ToolExecution`/`ToolResult`, tool-presentation UI types, the `tools/execute` waterfall |
 | [bash.md](bash.md) | the bash executor seam: `BashExecRequest`/`Spec`, `BashRunResult`, background `BashTask`s |
+| [subagent.md](subagent.md) | the subagent seam: the named-provider registry, `SubagentStartRequest`/`Result`/`Run`, the start-time-vs-runtime capability split |
 
 > Type definitions on this page are pasted **verbatim** from source and drift-checked by `pnpm run verify-type-equiv` (see [development.md](../development.md#documenting-types-verbatim-ts-type-equiv)). Inline JSDoc is omitted for readability; follow the source link for the full contracts.