Replace upgrade.sh with Go-based upgrade tool in tools/upgrade/

Rewrite the bash upgrade script as a standalone Go program for better
portability and maintainability. The tool preserves all existing behavior:
three-category file classification, dry-run mode, Go source resolution
(gvm/GOROOT/download), 3-way merge via diff3, and colored output.

Build: cd tools/upgrade && go build -o upgrade .
Run:   ./tools/upgrade/upgrade --dry-run

Signed-off-by: deadprogram <ron@hybridgroup.com>

.gitignore

@@ -23,3 +23,5 @@ go.work
 # Upgrade script work/report directories
 .upgrade-work/
 .upgrade-report/
+
+./tools/upgrade/upgrade