Prepare for npm publish as @protobi/exceljs with sunset plan

Updates fork documentation to clarify this is a temporary bridge fork
with an explicit plan to sunset once upstream merges our contributions.

Changes:
- FORK.md: Complete rewrite with PR tracking table and migration plan
  * Tracks 6 adopted PRs awaiting upstream merge
  * Lists 3 original features ready to submit upstream
  * Provides deprecation process and migration instructions
  * API compatibility guarantees for easy switching

- README.md: Add prominent fork notice at top
  * Clear warning this is temporary fork
  * Installation instructions for @protobi/exceljs
  * Migration path back to official package
  * Links to FORK.md for detailed status

- package.json: Prepare for npm publishing
  * Update name: exceljs → @protobi/exceljs
  * Bump version: 4.4.0-protobi.1 → 4.4.0-protobi.2
  * Update description to mention temporary nature
  * Add funding field pointing to upstream sponsors
  * Add fork-related keywords (pivot-tables, fork, temporary-fork)

Goal: Publish to npm as bridge solution while actively working to
merge all changes upstream and sunset this fork.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: protobi-pieter