Standardize all SDC components to one authoring convention (attributes / classes / structure), add AGENTS.md, and fix related component bugs

Problem/Motivation

The theme components follow different authoring patterns, which makes them harder to maintain and to keep consistent.

Proposed resolution

Standardize all components to one authoring convention for attributes, classes, and structure, add an AGENTS.md guide, and fix the related component bugs found along the way.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ✅ Code review from 1 Varbase core team member
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes
• ✅ Release varbase-11.0.0-alpha3, varbase_starter-1.0.0-alpha2, vartheme_bs5-5.0.0-alpha3

Varbase update type

• ✅ No Update
• ➖ Optional Update
• ➖ Forced Update
• ➖ Forced Update if Unchanged

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• task: #3600830 Standardize all SDC components to one authoring convention and add AGENTS.md