spawnSync with shell:true fails on Windows network drives (UNC paths)

Bug

collectReviewContext() in lib/git.mjs calls getRepoRoot() → git rev-parse --show-toplevel, which on Windows network/NAS drives returns a UNC path (e.g. \192.168.8.4\NAS\AIProjects\MyRepo). This UNC path is then passed as cwd to spawnSync({ shell: true }) in lib/process.mjs, which invokes cmd.exe — and cmd.exe rejects UNC working directories:

git merge-base HEAD main: exit=128: '\192.168.8.4\NAS\AIProjects\MyRepo'
CMD.EXE was started with the above path as the current directory.
UNC paths are not supported.  Defaulting to Windows directory.
fatal: not a git repository (or any of the parent directories): .git

Root cause

In lib/process.mjs:

shell: process.platform === "win32"

When shell: true, Node.js uses cmd.exe on Windows. cmd.exe cannot use UNC paths as working directories.

The initial git commands succeed because process.cwd() returns the mapped drive letter (e.g. N:\AIProjects\MyRepo). But getRepoRoot() in lib/git.mjs:30 runs git rev-parse --show-toplevel, which resolves to the underlying UNC path. That UNC path is then used as cwd for all subsequent git calls in collectBranchContext() and collectWorkingTreeContext(), which fail.

Reproduction

1. Have a git repo on a Windows SMB/NAS share (either mapped drive like N: or direct UNC path)
2. Run any /codex:review or /codex:adversarial-review command from that repo

Suggested fixes (either would work)

• Use shell: false in spawnSync — git doesn't need a shell to execute, and this sidesteps the cmd.exe UNC limitation entirely
• Fall back to process.cwd() when git rev-parse --show-toplevel returns a UNC path on Windows (i.e. starts with \ or //)

Workarounds attempted (none worked)

• GIT_WORK_TREE / GIT_DIR environment variables — git still resolves to UNC
• git config --local core.worktree "N:/..." — --show-toplevel ignores it
• subst virtual drive — git resolves through to the real UNC path
• Overriding ComSpec to bash — not picked up correctly by Node.js
• PowerShell as launcher — same result, Node still spawns cmd.exe internally

Environment

• Windows 11 Pro
• Node.js (via Claude Code plugin runtime)
• Git for Windows
• SMB network share (Synology NAS)
• codex-plugin-cc v1.0.1

Start with lib/process.mjs to inspect the Windows spawnSync options, then trace the repository-root handling in lib/git.mjs and the calls from collectReviewContext(), collectBranchContext(), and collectWorkingTreeContext(). Reproduce the review command from a Windows repository on a mapped or UNC network drive, and consider the issue's proposed approaches; done means subsequent git commands succeed with the repository root as their working directory.