Establish fff-plus fork boundary

[vinitkumar]

Summary

• Rename the fork identity in docs to fff-plus.nvim
• Move fork-owned pickers under lua/fff/pickers with compatibility shims
• Point the Neovim binary downloader at vinitkumar/fff-plus.nvim release assets
• Adjust release workflow so the fork publishes GitHub release binaries without attempting upstream package publishing

Verification

• ruby -e "require 'yaml'; YAML.load_file('.github/workflows/release.yaml')"
• luajit -bl lua/fff/download.lua
• luajit -bl lua/fff/conf.lua
• luajit -bl lua/fff/main.lua
• luajit -bl lua/fff/pickers/init.lua
• luajit -bl lua/fff/pickers/{buffers,colors,git_files}.lua
• cargo build --release -p fff-nvim --features zlob
• nvim --headless -u NONE -c 'set rtp+=.' -c "lua local conf=require('fff.conf').get(); assert(conf.download.github_repo == 'vinitkumar/fff-plus.nvim'); require('fff.download'); require('fff.pickers'); require('fff.main'); require('fff.pickers.buffers'); require('fff.pickers.colors'); require('fff.pickers.git_files'); require('fff.buffers'); require('fff.colors'); require('fff.git_files')" -c 'qa'
• nvim --headless -u tests/minimal_init.lua -l tests/test_git_files.lua