Rewrite upstream PR #49: configurable fzf options

[xxxbrian]

Summary

• rework the useful part of upstream PR Allow override of rm and custom fzf options gazorby/fifc#49 on top of current main
• support global custom fzf options without mutating user config at runtime

What changed

• keep fifc_custom_fzf_opts as user config
• compute fifc_effective_fzf_opts per completion run in _fifc
• apply per-rule overrides in _fifc_action without writing back into persistent config
• document fifc_custom_fzf_opts in README.md

Why this is a rewrite

• upstream PR Allow override of rm and custom fzf options gazorby/fifc#49 bundled keybinding cleanup with config overrides
• this fork already merged the keybinding cleanup separately via upstream PR fix: remove extra tab binding gazorby/fifc#61
• this rewrite keeps only the configurable fzf behavior and drops the rm override entirely

Source

• upstream PR: Allow override of rm and custom fzf options gazorby/fifc#49