Add which-key and nvim-web-devicons, tidy brew casks

which-key needs an icon backend and treats mini.icons and
nvim-web-devicons as equally viable optional deps, so devicons covers
it. Both are pure APIs with no visible effect on their own; devicons
self-initialises on require, so no setup() call.

Group labels for the <leader>h and <leader>t prefixes live in the
which-key config rather than next to the gitsigns maps they describe,
keeping which-key's config in one place. Worth moving if more plugins
start contributing prefixes.

The Nerd Font the glyphs need was installed by hand on this machine
but missing from brew.sh, so a fresh bootstrap would have rendered
every icon as tofu. Records it as a cask alongside the ghostty
font-family setting that already expects it.

Also drops the claude-code cask, which was never actually installed
here: brew tracks stable releases, and we want newer ones, so it comes
from Claude's own installer instead. Left a comment so it doesn't get
re-added.

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

neovim/config/plugins/init.lua

@@ -1 +1,3 @@
 require('config.plugins.gitsigns')
+require('config.plugins.web-devicons')
+require('config.plugins.which-key')