feat(agent): add the agent/request-messages request-only message seam

A new waterfall near request construction lets plugins contribute
request-ONLY messages framing the derived history: RequestMessages
{ before, after } with a frozen empty seed, fired inside the open step
after the agent/request config waterfall, so the step/start boundary
snapshot and its same-sync-frame invariant are untouched. The request
becomes messagePrefix + boundary snapshot + messageSuffix.

Contributions never enter session history — deriveMessages() is
unchanged — so the request header is their durable record:
EpochHeader gains messagePrefix/messageSuffix (canonical absence for
empty arrays), request/header-delta replaces either array whole with
an empty array encoding the transition back to absence, and the
dev-mode reconstruction cross-check now expects the folded header's
framing around the boundary derivation.

This is the seam for per-request advisory context that must be
model-visible now without becoming durable history (a skills catalog,
an environment reminder), keeping the base system prompt
workspace-independent and provider prefix caches stable. The docs
carry the channel cost model: session-frozen content belongs in
before, low-frequency change notices belong in durable history via
inject() (paid once, prefix-cached thereafter), and after is reserved
for small frequently-refreshed state snapshots re-paid on every
request they ride. No shipped producer yet, so ACP snapshot fixtures
are byte-identical.

scripts/type-equiv.manifest.json

@@ -15,6 +15,8 @@
     { "doc": "docs/core-data-structures/core.md", "symbol": "PromptDecision", "source": "packages/core/agent/src/types.ts" },
     { "doc": "docs/core-data-structures/core.md", "symbol": "ContinuationDecision", "source": "packages/core/agent/src/types.ts" },
     { "doc": "docs/core-data-structures/core.md", "symbol": "SessionStartSource", "source": "packages/core/agent/src/types.ts" },
+    { "doc": "docs/core-data-structures/core.md", "symbol": "RequestMessages", "source": "packages/core/agent/src/types.ts" },
+    { "doc": "docs/core-data-structures/core.md", "symbol": "RequestMessagesContext", "source": "packages/core/agent/src/types.ts" },
 
     { "doc": "docs/core-data-structures/llm-streaming.md", "symbol": "StreamChunk", "source": "packages/llm/llm/src/types.ts" },
     { "doc": "docs/core-data-structures/llm-streaming.md", "symbol": "TokenUsage", "source": "packages/llm/llm/src/types.ts" },