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.

.github/copilot-instructions.md

@@ -6,8 +6,7 @@ This file exists to satisfy GitHub Copilot's convention of looking for instructi
 
 ## Quick Reference
 
-- **Primary instructions**: See `/home/ray/.config/nvim/AGENTS.md`
-- **Migration plan**: See `MIGRATION_PLAN.md`
+- **Primary instructions**: See `AGENTS.md`
 - **Decision log**: See `LOG.md`
 - **Keymap reference**: See `README.md`
 
@@ -15,9 +14,8 @@ This file exists to satisfy GitHub Copilot's convention of looking for instructi
 
 1. **Read `AGENTS.md` first** - it contains all coding rules, architecture decisions, and what NOT to reintroduce
 2. **Update docs after changes**:
-   - Config changes → update `MIGRATION_PLAN.md`
    - Decisions → update `LOG.md`
-   - Keymap changes → update `README.md`
+   - User-facing behavior and keymaps → update `README.md`
 3. **Modern Lua-first**: Use `vim.opt`, `vim.keymap.set`, `vim.api` - avoid Vimscript
 4. **LSP (Neovim 0.11+)**: Use `vim.lsp.config()` + `vim.lsp.enable()`, NOT lspconfig `.setup()`
 5. **Project config**: Native `exrc` + `secure`, no neoconf plugin