fix: detect VS Code Copilot via CLAUDE_PLUGIN_ROOT fallback (#528) (#579)

VS Code Copilot never sets COPILOT_PLUGIN_DATA, only CLAUDE_PLUGIN_ROOT
(pointed at an .vscode/agent-plugins/... install path), so isCopilot was
false and ponytail assumed native Claude Code — emitting the statusline
setup nudge, which VS Code Copilot doesn't read. Also guard stateDir:
it was built from the same unset COPILOT_PLUGIN_DATA whenever isCopilot
resolved via this new fallback, so fall back to getClaudeDir() there too.