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/src/index.ts

@@ -0,0 +1,49 @@
+import { Context, Service } from 'cordis'
+import type { Agent } from './types.ts'
+
+export * from './types.ts'
+
+declare module 'cordis' {
+  interface Context {
+    agents: AgentRegistry
+  }
+}
+
+/**
+ * Agent registry (`ctx.agents`): tracks live agents so UI, hook, and
+ * orchestrator plugins can find them without depending on the concrete loop
+ * package. Agent *creation* belongs to whichever plugin implements the Agent
+ * interface (phase 1: `@deepseek-ai/dsh-agent-loop`).
+ */
+export class AgentRegistry extends Service {
+  private store = new Map<string, Agent>()
+
+  constructor(ctx: Context) {
+    super(ctx, 'agents')
+  }
+
+  /** Register a live agent. Disposed with the calling fiber. */
+  register(agent: Agent): () => void {
+    return this.ctx.effect(() => {
+      if (this.store.has(agent.id)) {
+        throw new Error(`agent "${agent.id}" is already registered`)
+      }
+      this.store.set(agent.id, agent)
+      this.ctx.emit('agent/created', agent)
+      return () => {
+        this.store.delete(agent.id)
+        this.ctx.emit('agent/disposed', agent)
+      }
+    }, 'agents.register()')
+  }
+
+  get(id: string): Agent | undefined {
+    return this.store.get(id)
+  }
+
+  list(): Agent[] {
+    return [...this.store.values()]
+  }
+}
+
+export default AgentRegistry