security(home): drop all neovim plugins to shrink supply-chain exposure (#259)

* chore(home): drop AstroNvim bundle and centralize editors

Remove the bundled Neovim plugin tree to shrink supply-chain exposure; keep a
minimal programs.neovim backup. Daily editing stays Helix-first with Yazi and
Zellij.

- Add Helix/Neovim docs, glossary, and cheatsheets under core/editors/.
- Default EDITOR/VISUAL to hx; use SUDO_EDITOR nvim --clean for sudoedit and
  other sensitive edits; adjust Nushell buffer_editor accordingly.
- Apply Helix Home Manager settings (keys, editor UI) in core/editors/helix.
- Keep heavy language-server tooling only in home/base/tui/editors/packages.nix;
  wire it via an explicit default.nix import so core stays lightweight.
- Extend modules/base packages.nix where that profile sets global editor env.

* docs: link editors, VS Code, and agents from README

Document tui/editors versus core/editors; add root README pointers to GUI
vscode (Home Manager) and agents/. Cross-link heavy packages README from
core/editors.

home/base/core/editors/README.md

@@ -1,10 +1,26 @@
 # Editors
 
-This directory contains editor configurations that are shared across different environments.
+Shared editor configuration and **usage notes** for terminal-focused editing.
 
-## Available Editors
+## Roles
 
-- **neovim/**: Neovim configuration with AstroNvim
-- **helix/**: Helix editor configuration
+- **Helix** (`helix/`): Primary TUI editor — batteries-included, small attack surface. `$EDITOR` /
+  `$VISUAL` default to `hx` (`session-env.nix`).
+- **Neovim** (`neovim/`): Backup editor — classic vim-style workflow and `:help` when needed. For
+  privileged edits (`sudoedit`) and other security-sensitive inputs, `$SUDO_EDITOR` is
+  `nvim --clean`; use that explicitly when `$EDITOR` must avoid user config/plugins.
 
-These configurations are designed to work across both terminal and GUI environments.
+Terminal layout and files: **Zellij** and **Yazi** live under `core/zellij/` and `core/yazi.nix`
+(not in this folder).
+
+## Docs
+
+- [`helix/README.md`](./helix/README.md) — Helix basics, cheatsheet, official doc links, Helix vs
+  Neovim notes.
+- [`neovim/README.md`](./neovim/README.md) — Vim/Neovim basics, cheatsheet, and official doc links.
+
+Nix modules in `helix/` and `neovim/` enable each editor via Home Manager. Language servers and
+other heavy editor-related packages are listed in
+[`../../tui/editors/`](../../tui/editors/README.md) (imported from `home/base/tui`).
+
+Editor terminology (LSP, tree-sitter): [`Glossary.md`](./Glossary.md).