fix(ci): fix warning annotation and guard semantic-release on Go test success

Send ::warning:: to stdout (not stderr) so GitHub Actions renders it as
an annotation. Gate semantic-release on test and nix-build success when
Go changes are present, preventing releases without passing tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: cpcloud

.github/detect-go-changes.bash

@@ -32,7 +32,7 @@ fi
 if [ "$event_name" != "pull_request" ]; then
   file_count=$(echo "$changed" | wc -l)
   if [ "$file_count" -ge 300 ]; then
-    echo "::warning::Compare API file cap hit ($file_count files), assuming Go changes" >&2
+    echo "::warning::Compare API file cap hit ($file_count files), assuming Go changes"
     echo true
     exit 0
   fi

.github/workflows/ci.yml

@@ -231,6 +231,7 @@ jobs:
       always()
       && !contains(needs.*.result, 'failure')
       && !contains(needs.*.result, 'cancelled')
+      && (needs.changes.outputs.go != 'true' || (needs.test.result == 'success' && needs.nix-build.result == 'success'))
     runs-on: ubuntu-latest
     concurrency:
       group: semantic-release-${{ github.ref }}