feat(git): replace vim-fugitive with custom git module

after/ftplugin/git.lua

@@ -0,0 +1,17 @@
+-- The built-in `git` filetype is reused for our `:G show` / `:G cat-file -p`
+-- output buffers and for commits opened from the log window. We set
+-- `vim.b.git_worktree` before assigning the filetype on those buffers; the
+-- guard below keeps the <CR> dispatcher off any unrelated git buffer (a
+-- real `.git/HEAD` file, etc.) so the default normal-mode <CR> still works.
+if not vim.b.git_worktree then
+    return
+end
+
+local cr = vim.api.nvim_replace_termcodes("<CR>", true, false, true)
+
+vim.keymap.set("n", "<CR>", function()
+    if not require("git.show").open_at_cursor() then
+        -- "n" mode = no remap, so this doesn't recurse into our mapping.
+        vim.api.nvim_feedkeys(cr, "n", false)
+    end
+end, { buffer = 0, silent = true, desc = "Open file at commit" })