Document fork changes in FORK_CHANGES.md

[ShlomoStept]

Goal

Create FORK_CHANGES.md documenting this fork's enhancements vs upstream.

Include

• Foundation features (ANSI, content blocks, syntax highlighting, copy buttons)
• Advanced UX (collapsible cells, tool/result pairing, markdown-in-tools, metadata, icons)
• Bug fixes (thread safety, clipboard fallback, duplicate test fix, view toggles)
• Test coverage summary

Acceptance Criteria

• FORK_CHANGES.md exists at repo root
• Sections clearly map features to behavior and tests