Persist js_repl codex helpers across cells

[fjord-oai]

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.

[fjord-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Another round soon, please!

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