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/support/invariants/src/index.ts

@@ -368,7 +368,7 @@ export function apply(ctx: Context, config: Config = {}): void {
   // be EXACTLY what the session log reconstructs:
   //
   // - messages: the folded header's request-only messages (messagePrefix /
-  //   messageSuffix — the `agent/request-messages` contributions, logged on
+  //   messageSuffix — the `agent/request-advice` contributions, logged on
   //   the header because no session event carries them) framing the
   //   derivation over the log prefix strictly before the in-flight step's
   //   `step/start` (the reconstruction boundary). The derivation is compared