docs(events): address Codex round-2 — purge remaining single-waterfall references

Codex's confirmation review confirmed blocker #1 (post-execute mutation) fixed and
the guard real, but found three current-state references the first sweep missed
(they phrase the seam without the literal "tools/execute" string):

- packages/core/tools/src/index.ts: the ToolExecution JSDoc said "flows through the
  execution waterfall" → now names the pre-execute → dispatch → post-execute pipeline.
- packages/core/tools/src/schema.ts: ToolArgsError's JSDoc said "the registry's
  execute waterfall catches it" → "the registry's execution pipeline".
- AGENTS.md repo layout still described tools/ as "tool registry + tools/execute
  waterfall" → "tools/pre-execute/post-execute pipeline".

Doc-comment-only; no behavior change.

packages/core/tools/src/index.ts

@@ -224,7 +224,7 @@ export interface ToolResult {
   isError: boolean
 }
 
-/** One pending tool call, as it flows through the execution waterfall. */
+/** One pending tool call, as it flows through the execution pipeline (`tools/pre-execute` → dispatch → `tools/post-execute`). */
 export interface ToolExecution {
   callId: CallId
   name: string