docs(agent): sync inject wording + resume error wording with code (review #34)

- agent/README: the inject() line said "without triggering a turn",
  regressing the #32 turn-enclosure model. Restored the running-vs-idle
  wording (idle inject wraps a one-shot injection turn; ADR 0017) to match
  the interface JSDoc.
- agent-loop resume() JSDoc said "throws a typed error" but the code
  throws a plain Error (consistent with the sibling assertAgentIdFree
  throw). Softened to "rejects with a clear error" — no behavior change;
  plain Error is intentional (no consumer needs a structured code here).

packages/agent-loop/src/index.ts

@@ -114,7 +114,7 @@ export class AgentLoop extends Service implements AgentFactory {
    * continue), and starts a fresh agent on it. The live session id is the
    * resumed id, NOT `${agentId}-session`.
    *
-   * Requires `ctx.sessionPersistence`; throws a typed error if it is not
+   * Requires `ctx.sessionPersistence`; rejects with a clear error if it is not
    * configured. NOT hard-injected (that would make non-persistent demos pend
    * forever) — callers that need resume (ACP) inject `sessionPersistence`, so
    * by the time this runs the service exists.