docs(hooks): correct fold description + drop history-narrating test comments

Codex convergence findings on the delegate-and-fold fix (code path verified
correct, prose only):

- The hook-bridges RFC claimed a downstream `block` "carries the bridge context
  too" for BOTH seams. True for `tools/post-execute` (PostToolDecision.block has
  an additionalContext field) but false for `agent/prompt-submit`
  (PromptDecision.block is `{kind,reason}` with no context field). The code is
  already correct — a blocked prompt drops the context, which is right since the
  prompt never reaches the model. Reworded the RFC to state the per-seam
  difference accurately.
- Two test comments narrated "Before the fix…", which the current-state-only
  doc rule forbids. Reworded to describe the behavior, not its history.
- Documented on concatContext (both bridges) why the merged block carries a
  single source: a HookContext holds one MessageSource and the seam cannot
  represent mixed provenance; rendering distinguishes only by source.kind, so a
  downstream plugin's text stays framed as plugin context.

packages/hooks/hooks-codex/src/index.ts

@@ -169,7 +169,11 @@ export function apply(ctx: Context, config: Config): void {
   /**
    * Concatenate this bridge's {@link HookContext} (`ours`, always present at the
    * call sites) with a downstream listener's optional one, so folding our
-   * additionalContext onto a delegated decision drops neither.
+   * additionalContext onto a delegated decision drops neither. The merged block
+   * carries a single `source` — this bridge's — because a `HookContext` holds one
+   * `MessageSource` and the seam cannot represent mixed provenance; the rendered
+   * `context/message` only distinguishes by `source.kind` ('plugin'), so a
+   * downstream plugin's text is still correctly framed as plugin context.
    */
   function concatContext(ours: HookContext, theirs: HookContext | undefined): HookContext {
     if (!theirs) return ours