Add subagent capability seam: interface, mock backend, model-facing tool

Introduce the `packages/subagent/` group and the abstract subagent seam — an
agent delegating to a child agent — as a named-provider registry (`ctx.subagents`),
unlike the single-implementation bash seam, so multiple transports (in-process,
ACP, future A2A) coexist. This first PR lands the interface, a scripted test
backend, and the model-facing tool, validated through the real cordis load path.

- dsh-subagent: SubagentService registry + SubagentProvider/SubagentRun
  vocabulary + subagent/start|end events. Start-time capabilities (outputSchema,
  depthLimit, toolFilter) are checked pre-start and rejected loud; runtime
  capabilities (sendMessage, resume) are optional methods on SubagentRun.
- dsh-subagent-mock (support): scripted provider for keyless, deterministic
  tests through the real Loader/export path.
- dsh-tool-subagent: the model-facing `subagent` tool, config-bound to one
  provider; synchronous collect with try/finally dispose, signal->cancel
  bridging, and non-completed-stop-reason -> isError mapping.
- Proposed RFC documenting the seam, the fork-vs-spawn-as-separate-backends
  decision, own-session isolation, synchronous-collect scope, and the deferral
  of background/poll/spill to a future unification with bash.
- Wire the new group into tsconfigs, build refs, package hierarchy docs, the
  module graph, and the cordis catalog.

RFC: docs/rfc/proposed/feature/2026-06-21-subagent-capability-seam.md

docs/module-graph.md

@@ -45,6 +45,9 @@ graph TD
   agent-loop --> session-persistence
   agent-loop --> system-prompt
   agent-loop --> tools
+  subagent --> agent
+  subagent --> llm
+  subagent --> tools
   tool-bash --> agent
   tool-bash --> bash
   tool-bash --> llm
@@ -57,6 +60,13 @@ graph TD
   agent-core --> system-prompt
   agent-core --> tool-bash
   agent-core --> tools
+  subagent-mock --> agent
+  subagent-mock --> llm
+  subagent-mock --> subagent
+  tool-subagent --> agent
+  tool-subagent --> llm
+  tool-subagent --> subagent
+  tool-subagent --> tools
   acp-agent --> acp
   acp-agent --> agent-core
   acp-agent --> session-persistence-jsonl
@@ -87,7 +97,10 @@ graph TD
 | `ui-stdio` | `agent`, `llm`, `session` |
 | `acp` | `agent`, `llm`, `session`, `session-persistence`, `tools` |
 | `agent-loop` | `agent`, `llm`, `session`, `session-persistence`, `system-prompt`, `tools` |
+| `subagent` | `agent`, `llm`, `tools` |
 | `tool-bash` | `agent`, `bash`, `llm`, `tools` |
 | `agent-core` | `agent`, `agent-loop`, `invariants`, `llm`, `session`, `system-prompt`, `tool-bash`, `tools` |
+| `subagent-mock` | `agent`, `llm`, `subagent` |
+| `tool-subagent` | `agent`, `llm`, `subagent`, `tools` |
 | `acp-agent` | `acp`, `agent-core`, `session-persistence-jsonl` |
 | `stdio-agent` | `agent`, `agent-core`, `session`, `session-persistence-jsonl`, `ui-stdio` |