## Summary
This changes `js_repl` so saved references to `codex.tool(...)` and
`codex.emitImage(...)` keep working across cells.
Previously, those helpers were recreated per exec and captured that
exec's `message.id`. If a persisted object or saved closure reused an
old helper in a later cell, the nested tool/image call could fail with
`js_repl exec context not found`.
This patch:
- keeps stable `codex.tool` and `codex.emitImage` helper identities in
the kernel
- resolves the current exec dynamically at call time using
`AsyncLocalStorage`
- adds regression coverage for persisted helper references across cells
- updates the js_repl docs and project-doc instructions to describe the
new behavior and its limits
## Why
We already support persistent top-level bindings across `js_repl` cells,
so persisted objects should be able to reuse `codex` helpers in later
active cells. The bug was that helper identity was exec-scoped, not
kernel-scoped.
Using `AsyncLocalStorage` fixes the cross-cell reuse case without
falling back to a single global active exec that could accidentally
attribute stale background callbacks to the wrong cell.
b560494c9f