Extract the shared in-process driver into dsh-subagent-inprocess (review feedback)

The shared run driver lived inside dsh-subagent-spawn, so the spawn package
carried fork-aware seeding logic and dsh-subagent-fork depended backward on
dsh-subagent-spawn — the two in-process backends were not independent.

Move the driver (startInProcessRun, depthOf, SubagentDepthError,
InProcessRunOptions) into a new pure-library package
@deepseek-ai/dsh-subagent-inprocess that registers nothing. spawn and fork now
both depend only on that driver and neither knows about the other; spawn no
longer re-exports it and fork no longer imports from spawn.

Also wire BOTH backends in examples/coding-agent/cordis.yml (config-only): load
dsh-subagent-spawn + dsh-subagent-fork + two dsh-tool-subagent instances with
distinct toolNames (subagent → spawn, subagent_fork → fork), demonstrating that
exposing multiple transports needs no code change.

packages/subagent/subagent-inprocess/tests/subagent-inprocess.spec.ts

@@ -0,0 +1,85 @@
+import { describe, expect, it } from 'vitest'
+import { Context } from 'cordis'
+import LlmService from '@deepseek-ai/dsh-llm'
+import SessionStore from '@deepseek-ai/dsh-session'
+import SystemPrompt from '@deepseek-ai/dsh-system-prompt'
+import ToolRegistry from '@deepseek-ai/dsh-tools'
+import AgentRegistry, { AgentId, type Agent } from '@deepseek-ai/dsh-agent'
+import AgentLoop from '@deepseek-ai/dsh-agent-loop'
+import * as Invariants from '@deepseek-ai/dsh-invariants'
+import SubagentService from '@deepseek-ai/dsh-subagent'
+import { MockAdapter, textResponse } from '../../../core/agent-loop/tests/mock-adapter.ts'
+import { depthOf, SubagentDepthError, startInProcessRun } from '../src/index.ts'
+
+type Script = ConstructorParameters<typeof MockAdapter>[0]
+
+/**
+ * Drives the shared in-process run driver DIRECTLY (no provider package), so the
+ * driver's own contract — depth read/cap, the one-shot drive, the result read —
+ * is covered independently of which backend (spawn/fork) calls it. The only
+ * mocked boundary is the model; the real agent loop, SubagentService, and
+ * dsh-invariants are mounted, so a malformed child session log fails the test.
+ */
+async function setup(script: Script) {
+  const ctx = new Context()
+  await ctx.plugin(LlmService)
+  await ctx.plugin(SessionStore)
+  await ctx.plugin(SystemPrompt)
+  await ctx.plugin(ToolRegistry)
+  await ctx.plugin(AgentRegistry)
+  await ctx.plugin(Invariants)
+  await ctx.plugin(AgentLoop, { agents: [] })
+  await ctx.plugin(SubagentService)
+  ctx.llm.registerAdapter(['mock'], new MockAdapter(script))
+  const parent = ctx.agentLoop.create(AgentId('parent'), { model: 'mock' })
+  return { ctx, parent }
+}
+
+function text(blocks: { type: string; text?: string }[]): string {
+  return blocks.filter(b => b.type === 'text').map(b => b.text).join('')
+}
+
+describe('depthOf', () => {
+  it('reads 0 for an agent with no subagentDepth, the set value otherwise', async () => {
+    const { parent } = await setup([])
+    expect(depthOf(parent)).toBe(0)
+    const withDepth = { options: { subagentDepth: 3 } } as unknown as Agent
+    expect(depthOf(withDepth)).toBe(3)
+  })
+})
+
+describe('startInProcessRun', () => {
+  it('drives a fresh child (no seed) to completion and returns its output', async () => {
+    const { ctx, parent } = await setup([textResponse('driver child answer')])
+    const run = startInProcessRun(ctx, { prompt: [{ type: 'text', text: 'do X' }], parent }, { providerName: 'spawn' })
+    const result = await run.result
+    expect(result.stopReason).toBe('completed')
+    expect(text(result.output)).toBe('driver child answer')
+    expect(depthOf(ctx.agents.get(run.id)!)).toBe(1)
+    await run.dispose()
+  })
+
+  it('throws SubagentDepthError when the child would exceed maxDepth', async () => {
+    const { ctx, parent } = await setup([])
+    expect(() => startInProcessRun(ctx, { prompt: [{ type: 'text', text: 'p' }], parent, maxDepth: 0 }, { providerName: 'spawn' }))
+      .toThrow(SubagentDepthError)
+  })
+
+  it('seeds the child session when a seed is supplied', async () => {
+    // Drive the parent through one real turn, then seed the child with that
+    // completed-turn prefix — the child must SEE the parent's history but its
+    // result is scoped to its OWN events (not the seeded parent message).
+    const { ctx, parent } = await setup([textResponse('parent turn'), textResponse('seeded child reply')])
+    parent.send([{ type: 'text', text: 'parent q' }])
+    await parent.whenIdle()
+    const seed = parent.session.events.slice()
+    const run = startInProcessRun(ctx, { prompt: [{ type: 'text', text: 'child q' }], parent }, { providerName: 'fork', seed })
+    const result = await run.result
+    expect(result.stopReason).toBe('completed')
+    expect(text(result.output)).toBe('seeded child reply')
+    const child = ctx.agents.get(run.id)!
+    // The child inherited the parent's prefix.
+    expect(child.session.events.slice(0, seed.length).some(e => e.type === 'user/message')).toBe(true)
+    await run.dispose()
+  })
+})