Harden git clone argv and config file permissions

[andrew]

Two security fixes from a code review, both test-driven.

git clone argument injection: repo.CloneURL comes from the forge API and was passed directly to exec.CommandContext(ctx, "git", "clone", url). A malicious or compromised self-hosted instance could return a CloneURL like --upload-pack=... which git would parse as an option rather than a URL. The fix extracts a gitCloneArgs helper that inserts -- before the URL to terminate option parsing. Applied to all three call sites in repo create --clone, repo fork --clone, and repo clone.

Config file permissions: os.WriteFile(path, data, 0600) only sets the mode when creating a new file. If ~/.config/forge/config already exists with looser permissions (manual creation, backup restore), writing a token via SetDomain would not tighten it. Now writeINI calls os.Chmod after the write on non-Windows platforms.

Both tests fail on main and pass after the fix.