Add abstract service interface packages

@deepseek-ai/dsh-llm: provider-neutral content-block vocabulary
(merge-extensible maps), raw StreamChunk protocol, ToolSchema,
abstract LlmAdapter, LlmService adapter registry, BlockAssembler.

@deepseek-ai/dsh-session: event-sourced Session (append-only log,
deriveMessages; context/steering render as tagged envelopes),
SessionStore, session/event + awaited session/flush durability seam.

@deepseek-ai/dsh-system-prompt: ordered sections + tool-schema
providers; assemble() through the system-prompt/assemble waterfall.
Tool schemas are part of the assembly by design.

@deepseek-ai/dsh-tools: tool registry feeding schemas into the
assembly; execute() through the tools/execute waterfall (the single
sandbox/permission/hook seam).

@deepseek-ai/dsh-agent: Agent interface (send/steer/inject/abort,
spawn/fork TODO seams), AgentRegistry, and the full agent/* event
taxonomy so plugins never depend on the concrete loop.

packages/agent/tests/agent.spec.ts

@@ -0,0 +1,54 @@
+import { describe, expect, it } from 'vitest'
+import { Context } from 'cordis'
+import { Session } from '@deepseek-ai/dsh-session'
+import AgentRegistry, { Agent } from '@deepseek-ai/dsh-agent'
+
+function stubAgent(id: string): Agent {
+  return {
+    id,
+    options: {},
+    session: new Session(`${id}-session`),
+    status: 'idle',
+    send() {},
+    steer() {},
+    inject() {},
+    abort() {},
+  }
+}
+
+describe('AgentRegistry', () => {
+  it('registers agents and emits created/disposed events', async () => {
+    const ctx = new Context()
+    await ctx.plugin(AgentRegistry)
+
+    const created: string[] = []
+    const disposed: string[] = []
+    ctx.on('agent/created', agent => void created.push(agent.id))
+    ctx.on('agent/disposed', agent => void disposed.push(agent.id))
+
+    const agent = stubAgent('a1')
+    const dispose = ctx.agents.register(agent)
+    expect(created).toEqual(['a1'])
+    expect(ctx.agents.get('a1')).toBe(agent)
+    expect(ctx.agents.list()).toEqual([agent])
+
+    dispose()
+    expect(disposed).toEqual(['a1'])
+    expect(ctx.agents.get('a1')).toBeUndefined()
+  })
+
+  it('rejects duplicate ids and unregisters on fiber dispose (HMR safety)', async () => {
+    const ctx = new Context()
+    await ctx.plugin(AgentRegistry)
+    ctx.agents.register(stubAgent('main'))
+    expect(() => ctx.agents.register(stubAgent('main'))).toThrow('already registered')
+
+    const fiber = await ctx.plugin(Object.assign((inner: Context) => {
+      inner.agents.register(stubAgent('scoped'))
+    }, { inject: ['agents'] }))
+    expect(ctx.agents.list().map(a => a.id)).toEqual(['main', 'scoped'])
+
+    await fiber.dispose()
+    expect(ctx.agents.list().map(a => a.id)).toEqual(['main'])
+  })
+})