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.

AGENTS.md

@@ -59,7 +59,7 @@ packages/    Harness packages, grouped by role at packages/<group>/<pkg>/.
   core/           product API spine
     session/        event-sourced session log + in-memory store
     system-prompt/  prompt-section + tool-schema assembly registry
-    tools/          tool registry + tools/execute waterfall
+    tools/          tool registry + tools/pre-execute/post-execute pipeline
     agent/          Agent interface, registry, agent/* event vocabulary
     agent-loop/     THE concrete plugin: ReactLoopAgent + the loop driver
     agent-core/     bundle plugin: the providerless/executor-less/UI-less spine