fix: add shellcheck disable directives to check scripts

Add appropriate shellcheck disable comments to suppress false positives:
- SC2310: Functions in conditionals (intentional pattern)
- SC2312: Masked return values (intentional)
- SC2329: "Unused" functions (sourced by other scripts)
- SC1091: Can't follow source (file exists at runtime)

Author: mlwelles

t/scripts/check-ack.sh

@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+# shellcheck disable=SC2310,SC2329
 set -euo pipefail
 
 # shellcheck source=checkhelper.sh

t/scripts/check-docker.sh

@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+# shellcheck disable=SC1091,SC2310,SC2312,SC2329
 set -euo pipefail
 
 # shellcheck source=checkhelper.sh

t/scripts/check-go.sh

@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+# shellcheck disable=SC2310,SC2329
 set -euo pipefail
 
 # shellcheck source=checkhelper.sh

t/scripts/check-gotestsum.sh

@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+# shellcheck disable=SC2310
 set -euo pipefail
 
 # shellcheck source=checkhelper.sh

t/scripts/check-protoc.sh

@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+# shellcheck disable=SC2310,SC2329
 set -euo pipefail
 
 # shellcheck source=checkhelper.sh

t/scripts/checkhelper.sh

@@ -1,5 +1,7 @@
 #!/usr/bin/env bash
+# shellcheck disable=SC2312
 # Common functions for check scripts
+# This is a library sourced by other scripts - functions are invoked indirectly
 
 err() { printf "ERROR: %s\n" "$*" >&2; }
 warn() { printf "WARN:  %s\n" "$*" >&2; }