Add timeout to file watcher subscriptions

Improvements

• CoreAdjust git watcher to ignore files other than HEAD
• CoreSet small options for Google models on OpenRouter@neominik
• CoreAdd Nix syntax highlighting@anntnzrb
• CoreMake format parameter optional with markdown default in WebFetch@samcornor
• CoreFilter messages with only step-start parts in toModelMessage@coleleavitt
• CoreAdd nixfmt formatter for Nix files@anntnzrb
• DesktopFix error page formatting in desktop app
• DesktopImprove error messages in desktop app