CI: disable setup-go cache to silence go.sum warning

This module has zero external dependencies, so no go.sum exists and there is
nothing to cache. setup-go's cache defaults to hashing go.sum and emitted a
"Dependencies file is not found ... go.sum" restore-cache warning on every run.
Set cache: false with an explanatory note.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: waveriderai

.github/workflows/ci.yml

@@ -16,6 +16,10 @@ jobs:
         with:
           go-version: '1.22'
           check-latest: true
+          # No go.sum: this module has zero external dependencies, so there is
+          # nothing to cache. Disabling setup-go's cache silences the harmless
+          # "Dependencies file is not found ... go.sum" restore-cache warning.
+          cache: false
 
       - name: Build
         run: go build ./...