Revert "chore: bump Claude Code to 2.1.21 and Agent SDK to 0.2.21" (#869)

This reverts commit 49046e0709.

base-action/action.yml

@@ -124,7 +124,7 @@ runs:
         PATH_TO_CLAUDE_CODE_EXECUTABLE: ${{ inputs.path_to_claude_code_executable }}
       run: |
         if [ -z "$PATH_TO_CLAUDE_CODE_EXECUTABLE" ]; then
-          CLAUDE_CODE_VERSION="2.1.21"
+          CLAUDE_CODE_VERSION="2.1.20"
           echo "Installing Claude Code v${CLAUDE_CODE_VERSION}..."
           for attempt in 1 2 3; do
             echo "Installation attempt $attempt..."