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/core/session/src/index.ts

@@ -252,7 +252,11 @@ export class Session {
    */
   requestHeader(): EpochHeader | undefined {
     if (this.headerFoldSeq < this.log.length) {
-      this.headerFold = foldRequestHeader(this.log.slice(this.headerFoldSeq), this.headerFold)
+      // Frozen on update: the fold is session state exposed by reference — a
+      // consumer mutating it in place (instead of building a replacement)
+      // would desync every later comparison against the log, so mutation
+      // throws instead.
+      this.headerFold = deepFreeze(foldRequestHeader(this.log.slice(this.headerFoldSeq), this.headerFold))
       this.headerFoldSeq = this.log.length
     }
     return this.headerFold