feature/go-rewrite : base Go avec walk GUI

- Rewrite complet en Go : exe unique sans _internal/, sans extraction temp
- GUI Windows-native via github.com/lxn/walk (TableView, TextEdit, PushButton)
- Meme fonctionnalites : check repos, pull, checkout, auto-update, logs
- build.bat : go build -ldflags "-H windowsgui -s -w" -> 9.6 Mo, zero dependance

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

go.mod

@@ -0,0 +1,11 @@
+module gitchecker
+
+go 1.22
+
+require github.com/lxn/walk v0.0.0-20210112085537-c389da54e794
+
+require (
+	github.com/lxn/win v0.0.0-20210218163916-a377121e959e // indirect
+	golang.org/x/sys v0.18.0 // indirect
+	gopkg.in/Knetic/govaluate.v3 v3.0.0 // indirect
+)