feat(agent): create/resume factory seam

Add the agent-creation factory seam on ctx.agents (AgentRegistry):
setFactory/create/resume plus the AgentFactory interface and
CreateAgentOptions/ResumeAgentOptions. AgentLoop implements AgentFactory
and registers itself via ctx.agents.setFactory(this), so plugins
create/resume agents through the interface without depending on the
concrete loop package.

- create({ agentId, sessionId, meta?, agentOptions? }) — programmatic
  create on a caller-supplied session id (e.g. an ACP-generated id).
- resume({ agentId, resumeSessionId, agentOptions? }) — load a persisted
  session via ctx.sessionPersistence (RFC 009) and resume an agent on it;
  the live session id is the resumed id, turn numbering and derived
  history continue from the loaded log. sessionPersistence is NOT
  hard-injected (non-persistent demos still work); resume rejects with a
  typed error when it is absent. assertAgentIdFree runs before any
  session is created (and again after the load await) so a duplicate id
  never leaves an orphaned live session.

Adds the runtime dsh-session-persistence dependency to agent-loop.

packages/agent-loop/package.json

@@ -23,6 +23,7 @@
     "@deepseek-ai/dsh-agent": "^0.0.1",
     "@deepseek-ai/dsh-llm": "^0.0.1",
     "@deepseek-ai/dsh-session": "^0.0.1",
+    "@deepseek-ai/dsh-session-persistence": "^0.0.1",
     "@deepseek-ai/dsh-system-prompt": "^0.0.1",
     "@deepseek-ai/dsh-tools": "^0.0.1",
     "cordis": "^4.0.0-rc.6"
@@ -35,6 +36,7 @@
     "@deepseek-ai/dsh-invariants": "^0.0.1",
     "@deepseek-ai/dsh-llm": "^0.0.1",
     "@deepseek-ai/dsh-session": "^0.0.1",
+    "@deepseek-ai/dsh-session-persistence": "^0.0.1",
     "@deepseek-ai/dsh-session-persistence-jsonl": "^0.0.1",
     "@deepseek-ai/dsh-system-prompt": "^0.0.1",
     "@deepseek-ai/dsh-tools": "^0.0.1",