refactor(agent): replace the per-step advice seam with agent/session-prefix

Review discussion converged on the industry shape (Claude Code caches
user context per conversation; Codex separates initial context from
diffs; Kimi appends at continuation boundaries to protect prompt
caching): stable openers belong in a compose-once prefix, mid-session
changes belong in append-only history — not in a per-request slot.

agent/session-prefix fires ONCE per loop instance, lazily on its first
request-building step: the composed Message[] is deep-frozen, cached on
the transmission bookkeeping, recorded as EpochHeader.messagePrefix on
the anchoring 'initial'/'resume' snapshot, and reused verbatim for
every request the instance sends — prefix stability is structural, not
a producer discipline, and a resume recomposes with attributable drift.
The request is messagePrefix + boundary snapshot.

The per-step RequestAdvice/RequestAdviceContext surface and the
messageSuffix header field are dropped: the tail slot had no consumer,
and every current update pattern (new AGENTS.md discovered, memory
update, skills change) routes through the existing append-only history
channels — inject(), tools/post-execute additionalContext,
prompt-submit additionalContext — each paid once and prefix-cached
thereafter. The messagePrefix delta arm stays for codec totality; the
loop never produces one in practice.

packages/llm/llm/src/types.ts

@@ -175,8 +175,8 @@ export interface GenerateOptions {
   /**
    * Ordered conversation messages, exactly as the provider sees them (after
    * the `system` slot). A loop-built request assembles them as
-   * `EpochHeader.messagePrefix` + the derived history + `messageSuffix`
-   * (dsh-agent-loop); a hand-built one-shot passes any list.
+   * `EpochHeader.messagePrefix` + the derived history (dsh-agent-loop); a
+   * hand-built one-shot passes any list.
    */
   messages: Message[]
   /** System prompt text (adapters map to the provider's system slot). */