refactor(agent): rename agent/request-messages to agent/request-advice

tianyicui's review: the seam name did not say what the event or its
types do. 'advice' reads both ways — advisory content for the model,
and AOP before/after advice woven around a join point (here the
derived history) without modifying it — so RequestAdvice.before/after
are self-describing. Types follow: RequestAdvice / RequestAdviceContext;
the logged EpochHeader fields keep their positional names
(messagePrefix/messageSuffix).

Also sharpens the core.md wording the review flagged as ambiguous:
before-advice sits in front of the ENTIRE derived history, directly
after the system slot (the conventional home for session-stable openers
— an AGENTS.md digest, a skills catalog), after-advice follows the
history's last message. Catalogs and doc graphs regenerated.

packages/core/session/src/types.ts

@@ -203,7 +203,7 @@ export interface EpochHeader {
   tools?: ToolSchema[]
   /**
    * Request-only messages sent BEFORE the derived history (the
-   * `agent/request-messages` waterfall's `before` contributions). Not session
+   * `agent/request-advice` waterfall's `before` contributions). Not session
    * history — `deriveMessages()` never returns them — so the header is their
    * only durable record; absent when the request carried none.
    */