fix: drop commandWindows from hooks.json for Claude.ai marketplace validation (#593) (#601)

commandWindows is not part of the supported hooks schema for the Claude.ai
plugin marketplace validator, which rejects it as an unknown field. The
shared `command` field already runs cross-platform: Claude Code expands
${CLAUDE_PLUGIN_ROOT} before the shell sees it, VS Code Copilot ignores
commandWindows and runs `command` through PowerShell on Windows, and `node`
is natively available in both bash and PowerShell.

Update the regression test to assert commandWindows is absent and remove
the now-unused CMD_VAR_SYNTAX guard. Fix comment placeholder syntax
({{CLAUDE_PLUGIN_ROOT}} → ${CLAUDE_PLUGIN_ROOT}) and narrow test name.