feat(plugins): add codediff, tv, vellum plugins and drop overseer support

lua/plugins/codediff.lua

@@ -0,0 +1,13 @@
+--- codediff.nvim: VSCode-style diff review workspace (explorer + side-by-side)
+--- `:CodeDiff` reviews the working tree; `q` closes the review tab.
+--- @module plugins.codediff
+
+---@type LazySpec
+return {
+  "esmuellert/codediff.nvim",
+  cmd = "CodeDiff",
+  keys = {
+    { "<C-g><C-d>", "<Cmd>CodeDiff<CR>", desc = "Review changes (CodeDiff)" },
+  },
+  opts = {},
+}