[Windows] POSIX path conversion mangles slash-prefixed CLI args under Git Bash

Summary

On Windows, when the codex-companion runtime is invoked from a Git Bash
(MSYS2) shell, arguments that look like POSIX absolute paths (e.g. a PID
passed as /12345, or flags like /codex:cancel) get auto-translated by
MSYS's path-conversion layer into C:/Program Files/Git/12345, which
breaks the call.

Reproduction

Environment:

• Windows 10 / 11
• Git for Windows (MSYS2-based bash)
• @openai/codex-plugin-cc 1.0.5

Steps:

1. From Git Bash, start a background Codex job and obtain its job id.
2. Run /codex:cancel <pid> where <pid> is passed as /12345.
3. Observe that the call fails — the argument received by the runtime is
   C:/Program Files/Git/12345, not /12345.

Workaround

Set MSYS_NO_PATHCONV=1 in the user's shell profile (~/.bashrc for Git
Bash; $PROFILE for any PowerShell session that may spawn Git Bash
sub-shells). This globally disables MSYS path conversion for the session.

Suggested fix

Either:

• Have the plugin's wrapper script set MSYS_NO_PATHCONV=1 (or use the
  per-arg MSYS2_ARG_CONV_EXCL=* form) when spawning child processes on
  Windows, scoped narrowly so it does not pollute the parent shell, or
• Document the requirement explicitly in the README's Windows section.

Happy to send a PR if the maintainers prefer the wrapper-level fix.

Start by locating the plugin's Windows wrapper or child-process spawning path, then review the README's Windows section. Reproduce the Git Bash case with a slash-prefixed argument and determine whether the wrapper-level workaround or documentation is appropriate; done means /12345 and /codex:cancel are no longer mangled or the requirement is clearly documented.