gomod: drop stale golang.org/x/sync indirect line (closes #132)

go.mod listed golang.org/x/sync v0.19.0 twice: once in the direct
require block and once as // indirect. The module is a genuine direct
dependency (internal/portcheck/portcheck.go imports
golang.org/x/sync/errgroup), so the indirect entry was redundant and
stale. script/bootstrap ends with go mod download, which dropped that
line as a side effect and left every fresh checkout with a dirty
go.mod. Running go mod tidy removes the line for good; no dependency
and no version changed, and go.sum is unaffected.

Model: opus-4-8

TODO.md

@@ -23,6 +23,8 @@ Rationale, Design, TODO, License, Author) if any are still missing.
 
 # Completed Steps
 
+- 2026-09-21: `go mod tidy` dropped the redundant `golang.org/x/sync`
+  `// indirect` line so `script/bootstrap` leaves a clean tree (#132)
 - 2026-08-10: comment-only corrections to `script/bootstrap`,
   `script/cibuild`, and `Dockerfile.lint`. The `goimports` pin in
   `script/bootstrap` was justified by a claim that `script/fmt-check`

go.mod

@@ -40,7 +40,6 @@ require (
 	go.yaml.in/yaml/v2 v2.4.2 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
 	golang.org/x/mod v0.32.0 // indirect
-	golang.org/x/sync v0.19.0 // indirect
 	golang.org/x/sys v0.41.0 // indirect
 	golang.org/x/text v0.34.0 // indirect
 	golang.org/x/tools v0.41.0 // indirect