feat(agent): the agent is a registration scope — Agent.ctx, setup slot, fused scoped dispatch

Every live agent owns a dsh-scope context (Agent.ctx, key = the agent),
minted inside the loop's composite lifecycle effect: registrations through
it are agent-visible and agent-lifetime, and agent.ctx listeners hear only
that agent's dispatches. The composite yields the scope's raw disposer
first (identity-nested, no un-nested window), then session entry (scoped
enter captures the session carrier), then registration; teardown runs
stop/drain -> unregister -> detach session -> unwind scope, keeping
store/registry rollback synchronous on every failure path.

CreateAgentOptions.setup(agentCtx) runs after the scope is minted and the
agent registered, before agent/session-start and the loop start — the slot
where a creator composes the agent's scoped world (persona sections,
restrict(), scoped tools); a throwing setup unwinds inside the rollback
boundary. Setup registers, it never drives.

agentEvents(ctx, agent) fuses the scope carrier with the injected subject
argument for every agent/* dispatch (the correct dispatch is the shortest
spelling); assembleContextFor(agent) pairs the agent DX field with the
scope layer selector. All loop/agent/registry dispatch sites converted;
agent/* event declarations carry this: Scoped<Agent>; ctx.agent is a safe
root accessor defaulting undefined, shadowed by each agent context.

packages/core/agent/tests/agent.spec.ts

@@ -10,6 +10,9 @@ function stubAgent(rawId: string): Agent {
     options: {},
     session: new Session(SessionId(`${id}-session`)),
     status: 'idle',
+    // A bare context stands in for the agent scope: registry tests never
+    // register through it, they only need the field present.
+    ctx: new Context(),
     send() {},
     steer() {},
     inject() {},