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

packages/subagent/subagent/README.md

@@ -0,0 +1,39 @@
+# @deepseek-ai/dsh-subagent
+
+The **subagent seam**: an abstract `SubagentService` (`ctx.subagents`) for an agent delegating work to another agent. A *subagent* is a child agent; a `SubagentProvider` is one transport for running it.
+
+This package is the interface third of the capability seam, split so each concern evolves (and swaps) independently:
+
+| Package | Role |
+|---|---|
+| `@deepseek-ai/dsh-subagent` (this) | the interface: registry service + vocabulary types |
+| `@deepseek-ai/dsh-subagent-spawn` | an implementation: fresh in-process child |
+| `@deepseek-ai/dsh-subagent-fork` | an implementation: in-process child seeded from the parent's log |
+| `@deepseek-ai/dsh-subagent-acp` | an implementation: ACP client driving another process |
+| `@deepseek-ai/dsh-tool-subagent` | the model-facing tool over `ctx.subagents` |
+
+Unlike the bash seam (one executor per context, second load throws), **multiple providers coexist** here. Each registers under a unique name and a caller picks one by name — the shape mirrors the LLM adapter registry (`LlmService.registerAdapter`), not the single-service bash executor. This is the requirement that rules out the bash shape: an agent may want an in-process child for a cheap subtask and an out-of-process ACP child for an isolated one, in the same runtime.
+
+## Service API (`ctx.subagents`)
+
+| Member | Semantics |
+|---|---|
+| `registerProvider(provider)` | Register under `provider.name`. Throws `SubagentError('DUPLICATE_PROVIDER')` on a name clash. Effect-scoped (HMR-safe); returns the disposer. |
+| `getProvider(name)` | Look up a provider (`undefined` if absent). |
+| `list()` | Registered provider names (insertion order). |
+| `start(name, request)` | Resolve the provider (`NO_PROVIDER` if absent), validate every requested START-TIME capability (`UNSUPPORTED_CAPABILITY` for the first unmet one — before any child is created), then delegate to `provider.start` and emit `subagent/start` / `subagent/end` around the run. |
+
+## Capabilities: two kinds, discovered two ways
+
+- **Start-time features** (`outputSchema`, `depthLimit`, `toolFilter`) are a static `provider.capabilities` descriptor, checked by the service BEFORE a run exists. A request that needs one the provider lacks is **rejected loud** (`UNSUPPORTED_CAPABILITY`), never accepted-then-ignored.
+- **Runtime features** (steering, resume) are **optional methods** on `SubagentRun` (`sendMessage?`, `resume?`). The method's presence IS the capability; TS narrowing is the discovery mechanism — a consumer cannot call an absent method without narrowing first, so there is no silent degradation path.
+
+## Run lifecycle
+
+`provider.start(request)` returns a `SubagentRun`: a handle with a `result` promise, `cancel()`, `dispose()`, and the optional runtime methods. `result` resolves with a `SubagentResult` (`output`, optional `structured`, `stopReason`) — it does **not** reject on a child-level failure (a model/transport failure resolves with `stopReason: 'error'`), so the consumer maps a non-`completed` reason to an `isError` tool result. The consumer MUST `dispose()` on every path (success, error, abort) to reach child quiescence and avoid leaking an idle child / session.
+
+## Scope (first cut)
+
+The consumer collects **synchronously**: it starts a run and awaits `result`. Steering (`sendMessage`) is part of the contract but intentionally unused. Background / poll / spill semantics are deferred to a future redesign unifying long-running-tool handling across subagents and bash. See the RFC: [docs/rfc/proposed/feature/2026-06-21-subagent-capability-seam.md](../../../docs/rfc/proposed/feature/2026-06-21-subagent-capability-seam.md).
+
+See `src/types.ts` for the full contracts.