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.

docs/adr/README.md

@@ -27,5 +27,5 @@ Do NOT write an ADR for: a mechanical or local choice (a variable name, a one-fi
 | [0013](0013-property-based-testing.md) | Property-based testing for protocol-shaped code | accepted |
 | [0014](0014-doc-sync-enforcement.md) | Doc-sync enforcement (doc code blocks + event taxonomy) | accepted |
 | [0015](0015-structured-error-taxonomy.md) | Structured error taxonomy (HarnessError base) | accepted |
-| [0016](0016-session-persistence.md) | Session persistence as an abstract service over `SessionEvent` | accepted |
+| [0016](0016-session-persistence.md) | Session persistence as an abstract service over the existing `SessionEvent` | accepted |
 | [0017](0017-turn-enclosure-invariant.md) | Every session event is enclosed in a turn | accepted |