Replace diffview.nvim with codediff.nvim

diffview was doing the job for reviewing a branch before merging it, but
not well enough. codediff renders diffs the way VSCode does - light
line-level backgrounds with deeper character-level highlights on top of
them - so what actually changed within a line is legible rather than
inferred.

Keymaps carry over one for one: <leader>gd opens the status explorer,
<leader>gh/<leader>gH the file and branch history, and <leader>gm still
prompts for a base branch and diffs rev...HEAD against it. <leader>gc is
folded into codediff's own quit action alongside q, since there is no
close command to map it to.

Its diffing is done in C, and the library is downloaded prebuilt on
first use rather than compiled, so there is no build step for vim.pack
to worry about.

neovim/README.md

@@ -37,6 +37,12 @@ machine gets the same set.
 > nvim-web-devicons needs a Nerd Font in the terminal or the icons render as
 > tofu. `brew.sh` installs one for ghostty.
 
+> [!NOTE]
+> codediff.nvim does its diffing in C. It downloads a prebuilt library into its
+> own plugin directory on first use — so the first `:CodeDiff` on a new machine
+> needs network, and the binary is not covered by the lock file. `:CodeDiff
+> install!` forces a re-download if it ever goes missing or stale.
+
 ## Defaults worth knowing
 
 - **The colorscheme is catppuccin**, `flavour = 'auto'`: latte on a light