Add Go statusline implementation

Replace shell-based statusline with native Go for ~4x speedup.
Parses JSON natively, detects git status, checks gitea process,
and formats colored output with proper terminal width handling.

go.mod

@@ -1,3 +1,5 @@
 module github.com/kjanat/claude-statusline
 
 go 1.24.11
+
+require golang.org/x/sys v0.39.0