docs: update copilot instructions and README for v4.2/v4.3

[Copilot]

Sync both README.md and .github/copilot-instructions.md with the structural changes shipped in v4.2 and v4.3.

Block inventory

• Block count: 10 → 8 (asu-careers and image-gallery removed)
• Removed image-gallery/ from the src/ layout listing

Block architecture

• Added deprecated.js to the file-pattern table (backwards-compatible save() migrations)
• Updated hero example entry in the layout listing to include deprecated.js

Key patterns

• Save/Frontend pattern: clarified that most blocks use data-* attribute serialization, while hero (v3.0.0+) emits static HTML from save.js — frontend.js only activates for pre-v3 blocks via a dynamic import of legacy-hydration.js
• Deprecated saves: new bullet explaining when deprecated.js is required, pointing to src/hero/deprecated.js and src/news-grid/deprecated.js

Making changes guidance

• Added step to "Adding or modifying a block": include a deprecated.js entry whenever save.js output changes

Important notes

• Hero static HTML pattern + legacy hydration behaviour
• webpack.config.js disabling defaultVendors split-chunk cache group to keep async dynamic imports (e.g. hero/hero-legacy) self-contained