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

lua/plugins/vellum.lua

@@ -0,0 +1,19 @@
+--- vellum.nvim: live GitHub-markdown preview in a split, mermaid/math as images
+--- `:Vellum` toggles it, `:Vellum export [file.pdf|file.html]` exports.
+---
+--- Runs from the prdlk fork's `mdx` branch until blackhat-7/vellum.nvim#4
+--- merges: upstream only accepts filetype `markdown`, so `markdown.mdx`
+--- buffers could not export, take over the preview or be followed by links,
+--- and their import/export lines and {/* comments */} rendered as text.
+--- @module plugins.vellum
+
+---@type LazySpec
+return {
+  "prdlk/vellum.nvim",
+  branch = "mdx",
+  -- `markdown.mdx` is the mdx pack's filetype; a dotted filetype only matches
+  -- a pattern that spells it out
+  ft = { "markdown", "markdown.mdx" },
+  cmd = "Vellum",
+  opts = {},
+}