fix review findings: one frozen seed through the waterfall; stale agent/request docs

Codex diff review, round 1, two (A) findings:

- The agent/request fallback resolved the RAW seed object — on later
  steps the session's cached header fold — so a delegating listener
  (await next(), mutate, return) could rewrite the fold in place and
  the change would compare as already-baseline: no delta logged, the
  persisted log unable to reconstruct the request (the dev invariant
  would fire on the divergence, but the log would still lie). One
  structuredClone'd, deep-frozen seed now serves both the listener
  chain and the fallback — in-place shaping after delegation throws —
  and Session.requestHeader() freezes its fold on update, so the leak
  class is unrepresentable from either side. Pinned by a loop-level
  delegating-mutator test.
- Doc sweep for the old contract: agent README's event row (mutate
  GenerateOptions / tool filtering → frozen config seed, replacement
  out, logged header), compact-basic's module JSDoc (summarize routed
  through agent/request → direct one-shot at llm/stream), and
  architecture.md's event-domain line (request mutation → call-config
  shaping).

packages/compact/compact-basic/src/index.ts

@@ -9,9 +9,10 @@
  *   to the next balanced tool-pairing boundary so a compacted region never
  *   splits a step's tool-call/result pair (an open tail step is never crossed —
  *   compaction declines and retries once it closes).
- * - **Summarization** — `ctx.llm.stream()` assembled via `BlockAssembler`
- *   (the single model-call surface; same path the loop uses) with a fixed
- *   condense-the-history system prompt routed through `agent/request`.
+ * - **Summarization** — a direct one-shot `ctx.llm.stream()` call assembled
+ *   via `BlockAssembler` with a fixed condense-the-history system prompt;
+ *   NOT a loop step, so `agent/request` never fires — interception happens
+ *   at `llm/stream` like any other direct call.
  * - **Surface mutation** — a single `user/message` replace node carries the
  *   summary; `compact/*` events are log-only lock + provenance records.
  * - **Auto-compaction** — an `agent/pre-step` listener delegates to