Complete config migration cleanup

Retire legacy files and migration fixtures, move runtime state out of the repository, and migrate Tree-sitter and textobjects to their Neovim 0.12-compatible main branches.

lua/settings.lua

@@ -1,4 +1,4 @@
--- Minimal settings for bootstrap and Phase 3.2
+-- Core editor settings
 
 -- Disable unused language providers early; enable later only if a plugin requires them
 vim.g.loaded_ruby_provider = 0
@@ -64,7 +64,6 @@ utils.tabline_shorten_length = 4
 vim.opt.tabline = '%!v:lua.require("utils").custom_tabline()'
 vim.opt.showtabline = 1 -- Show tabline only when there are 2+ tabs
 
--- Phase 3.2: non-plugin settings (legacy values where specified)
 -- Native completion popup style
 vim.opt.pumblend = 0 -- No transparency
 
@@ -104,12 +103,12 @@ vim.opt.relativenumber = true
 -- Scrolloff settings
 vim.opt.scrolloff = 4
 
--- Phase 6.2: UX settings
+-- Editor UX
 vim.opt.signcolumn = "yes" -- Always show sign column (for LSP diagnostics, git signs, etc.)
 vim.opt.cursorline = true -- Highlight current line
 vim.opt.colorcolumn = "80,120" -- Visual guides at 80 and 120 characters
 
--- Phase 9.4: Default indentation (per-filetype overrides in after/ftplugin/)
+-- Default indentation (per-filetype overrides in after/ftplugin/)
 vim.opt.tabstop = 4 -- Display tabs as 4 spaces
 vim.opt.shiftwidth = 4 -- Indent by 4
 vim.opt.softtabstop = 4 -- Backspace removes 4 spaces
@@ -119,7 +118,9 @@ vim.opt.smartindent = true -- Smart autoindenting (C-like programs, PHP, etc.)
 
 -- Persistent undo
 vim.opt.undofile = true -- Enable persistent undo
-vim.opt.undodir = vim.fn.stdpath("config") .. "/undodir" -- Store undo files in config directory
+local undo_dir = vim.fn.stdpath("state") .. "/undo"
+vim.fn.mkdir(undo_dir, "p")
+vim.opt.undodir = undo_dir
 
 -- LSP diagnostics configuration
 vim.diagnostic.config({