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/tool-subagent/src/index.ts

@@ -0,0 +1,147 @@
+/**
+ * The model-facing `subagent` tool: delegate a task to a child agent and return
+ * its final output. Pure schema + lifecycle shaping — every transport concern
+ * lives behind the `ctx.subagents` provider registry
+ * (`@deepseek-ai/dsh-subagent`), so an in-process, ACP, or future A2A backend
+ * swaps in without touching what the model sees.
+ *
+ * Provider selection is config, not model-facing: this plugin is bound to
+ * EXACTLY ONE provider name (`Config.provider`). To expose more than one
+ * transport, load the plugin more than once, each bound to a different provider
+ * — there is no provider/type parameter in the model-facing schema. The model
+ * sees only `{ description, prompt }`.
+ *
+ * Collection is SYNCHRONOUS this cut: `execute` starts a run and awaits
+ * `run.result` inside a `try/finally` that always disposes the run, so the
+ * owned child agent/session is torn down on every path (success, error, abort)
+ * and never leaks as a live idle child. A non-`completed` stop reason maps to an
+ * `isError` tool result (by throwing) rather than returning partial output as
+ * success.
+ *
+ * @module @deepseek-ai/dsh-tool-subagent
+ */
+
+import type { Context } from 'cordis'
+import z from 'schemastery'
+import { defineTool } from '@deepseek-ai/dsh-tools'
+import type { AgentOptions } from '@deepseek-ai/dsh-agent'
+import type { ContentBlock } from '@deepseek-ai/dsh-llm'
+import type { SubagentResult, SubagentRun, SubagentStartRequest } from '@deepseek-ai/dsh-subagent'
+
+export const name = 'tool-subagent'
+export const inject = ['tools', 'subagents']
+
+/** Config: which registered provider this tool delegates to, plus child defaults. */
+export interface Config {
+  /** The `ctx.subagents` provider name to start runs on (e.g. `spawn`, `acp`). */
+  provider: 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.
+   */
+  agentOptions?: AgentOptions
+}
+
+export const Config: z<Config> = z.object({
+  provider: z.string().required(),
+  agentOptions: z.object({
+    model: z.string(),
+    systemPrompt: z.string(),
+  }),
+})
+
+/**
+ * Flatten a child's final output blocks to text for the tool result. The child
+ * may return non-text blocks; this cut surfaces the text content (the common
+ * case) and drops the rest, which is acceptable for a synchronous summary —
+ * the structured path (`outputSchema`) is the channel for non-text results.
+ */
+function outputText(blocks: ContentBlock[]): string {
+  return blocks
+    .filter((b): b is Extract<ContentBlock, { type: 'text' }> => b.type === 'text')
+    .map(b => b.text)
+    .join('')
+}
+
+/** A non-`completed` stop reason means the child did not finish cleanly. */
+function stopReasonError(result: SubagentResult): string | undefined {
+  switch (result.stopReason) {
+    case 'completed':
+      return undefined
+    case 'aborted':
+      return 'subagent run was cancelled'
+    case 'error':
+      return 'subagent run failed'
+    case 'max-tokens':
+      return 'subagent run hit its token limit before finishing'
+    case 'refusal':
+      return 'subagent declined the task'
+    // Merge-extensible union: a backend may add stop reasons. Treat an unknown
+    // terminal reason as a failure rather than reporting partial output as success.
+    default:
+      return `subagent run ended abnormally (${String(result.stopReason)})`
+  }
+}
+
+export function apply(ctx: Context, config: Config): void {
+  ctx.tools.register(defineTool({
+    name: '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 '
+      + 'implementation, an analysis — so it does not consume this conversation\'s context. The subagent '
+      + 'runs to completion and you receive only its final answer, not its intermediate steps. Give it a '
+      + 'complete, standalone prompt: it does not see this conversation.',
+    parameters: {
+      description: {
+        type: 'string',
+        required: true,
+        description: 'A short (3-5 word) description of the delegated task, for display.',
+      },
+      prompt: {
+        type: 'string',
+        required: true,
+        description: 'The complete, self-contained task for the subagent. It does not share this '
+          + 'conversation\'s context, so include everything it needs.',
+      },
+    },
+    async execute(args, exec): Promise<ContentBlock[]> {
+      const parent = exec.agent
+      if (!parent) {
+        // The loop sets `exec.agent` for every model-driven call; its absence
+        // means a non-agent caller invoked the tool directly, which has no
+        // parent to attribute the child to. Fail loud rather than guess.
+        throw new Error('subagent tool requires a calling agent (exec.agent was undefined)')
+      }
+
+      const request: SubagentStartRequest = {
+        prompt: [{ type: 'text', text: args.prompt }],
+        parent,
+        ...exec.signal ? { signal: exec.signal } : {},
+        ...config.agentOptions ? { agentOptions: config.agentOptions } : {},
+      }
+
+      const run: SubagentRun = ctx.subagents.start(config.provider, request)
+
+      // Bridge the tool's abort signal to the run: if the parent step is
+      // aborted while the child is in flight, cancel the child too.
+      const onAbort = (): void => { run.cancel('parent step aborted') }
+      exec.signal?.addEventListener('abort', onAbort, { once: true })
+
+      try {
+        const result = await run.result
+        const error = stopReasonError(result)
+        if (error !== undefined) {
+          // Map a non-clean finish to an isError result (the registry turns a
+          // throw into an isError). Report the reason, not partial output.
+          throw new Error(error)
+        }
+        return [{ type: 'text', text: outputText(result.output) }]
+      } finally {
+        exec.signal?.removeEventListener('abort', onAbort)
+        // Always reach child quiescence — never leak a live idle child/session.
+        await run.dispose()
+      }
+    },
+  }))
+}