Add js_repl host helpers and exec end events

[fjord-oai]

Summary

This PR adds host-integrated helper APIs for js_repl and updates model guidance so the agent can use them reliably.

What’s included

• Add codex.tool(name, args?) in the JS kernel so js_repl can call normal Codex tools.
• Keep persistent JS state and scratch-path helpers available:
  • codex.state
  • codex.tmpDir
• Wire js_repl tool calls through the standard tool router path.
• Add/align js_repl execution completion/end event behavior with existing tool logging patterns.
• Update dynamic prompt injection (project_doc) to document:
  • how to call codex.tool(...)
  • raw output behavior
  • image flow via view_image (codex.tmpDir + codex.tool("view_image", ...))
  • stdio safety guidance (console.log / codex.tool, avoid direct process.std*)

Why

• Standardize JS-side tool usage on codex.tool(...)
• Make js_repl behavior more consistent with existing tool execution and event/logging patterns.
• Give the model enough runtime guidance to use js_repl safely and effectively.

Testing

• Added/updated unit and runtime tests for:
  • codex.tool calls from js_repl (including shell/MCP paths)
  • image handoff flow via view_image
  • prompt-injection text for js_repl guidance
  • execution/end event behavior and related regression coverage

git stack

• ✅ 1 Add feature-gated freeform js_repl core runtime #10674
• 👉 2 Add js_repl host helpers and exec end events #10672
• ⏳ 3 Add js_repl_tools_only model and routing restrictions #10671
• ⏳ 4 Add feature-gated js_repl polling flow #10673
• ⏳ 5 Add optional js_repl isolation and vendored Node runtime #10670