[codex] add code mode io.write helper

[lt-oai]

What

• Adds a Code Mode io.write(value, "env://current/<relative-path>") helper.
• Routes writes through the Code Mode host so V8 stays filesystem-free.
• Supports text, byte arrays, MCP text blocks, and base64 blob/resource blocks.

Why

• Provides the harness-level write primitive needed for file handling workflows.
• Lets connector/tool outputs be materialized into the current environment without making each connector own filesystem writes.

How

• Adds an IoWrite runtime event and turn message.
• Implements core-side destination resolution, cwd escape checks, parent directory creation, and file writes.
• Documents the helper in the Code Mode tool description.

Validation

• cargo test -p codex-code-mode --lib -- --nocapture
• cargo test -p codex-core --test all code_mode_io_write_writes_text_to_current_env -- --nocapture