Add code style summary to agent build instructions

Added basic style guidelines for C++, Web UI, and CI/CD workflows.

Author: softhack007

.github/agent-build.instructions.md

@@ -20,6 +20,11 @@ Use these timeout values when running builds:
 
 ## Development Workflow
 
+### Code Style Summary
+- **C++** files in `wled00/` and `usermods/`: 2-space indentation (no tabs), camelCase functions/variables, PascalCase classes, UPPER_CASE macros. No C++ exceptions — use return codes and debug macros.
+- **Web UI** files in `wled00/data`: indent HTML and JavaScript with tabs, CSS with tabs.
+- **CI/CD workflows** in `.github/workflows`: 2-space indentation, descriptive `name:` on every workflow/job/step. Third-party actions must be pinned to a specific version tag — branch pins such as @main or @master are not allowed. SHA pinning recommended.
+
 ### Web UI Changes
 
 1. Edit files in `wled00/data/`