Add feature-gated freeform js_repl core runtime

[fjord-oai]

Summary

This PR adds an experimental, feature-gated js_repl core runtime so models can execute JavaScript in a persistent REPL context across tool calls.

The implementation integrates with existing feature gating, tool registration, prompt composition, config/schema docs, and tests.

What changed

• Added new experimental feature flag: features.js_repl.
• Added freeform js_repl tool and companion js_repl_reset tool.
• Gated tool availability behind Feature::JsRepl.
• Added conditional prompt-section injection for JS REPL instructions via marker-based prompt processing.
• Implemented JS REPL handlers, including freeform parsing and pragma support (timeout/reset controls).
• Added runtime resolution order for Node:
  1. CODEX_JS_REPL_NODE_PATH
  2. js_repl_node_path in config
  3. PATH
• Added JS runtime assets/version files and updated docs/schema.

Why

This enables richer agent workflows that require incremental JavaScript execution with preserved state, while keeping rollout safe behind an explicit feature flag.

Testing

Coverage includes:

• Feature-flag gating behavior for tool exposure.
• Freeform parser/pragma handling edge cases.
• Runtime behavior (state persistence across calls and top-level await support).

Usage

[features]
js_repl = true

Optional runtime override:

• CODEX_JS_REPL_NODE_PATH, or
• js_repl_node_path in config.

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