Add fzf-lua for project navigation

Wraps the fzf binary already installed via brew, alongside rg and fd,
so it needs no plugin dependencies - unlike telescope, which would
also pull in plenary. Picks up nvim-web-devicons for file icons, which
is the first thing actually using that plugin.

Maps live under <leader>f: files, live grep, buffers, grep the word
under the cursor, and help tags. The intent is to stop treating netrw
as the way into a project and switch to jumping by name, leaving
buffers open and bouncing between them, rather than quitting out of
nvim to change file.

Netrw is untouched and still on :Ex for browsing an unfamiliar tree.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

neovim/config/plugins/which-key.lua

@@ -7,6 +7,7 @@ wk.setup()
 -- Names for the <leader> prefixes, so the popup groups them sensibly
 wk.add({
   { '<leader>d', group = 'diagnostics' },
+  { '<leader>f', group = 'find' },
   { '<leader>h', group = 'hunks' },
   { '<leader>t', group = 'toggles' },
 })