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/README.md

@@ -15,7 +15,7 @@ home/
 │   │   ├── terminal/  # Terminal emulators (Kitty, Alacritty, etc.)
 │   │   └── ...
 │   ├── tui/           # Terminal/TUI applications
-│   │   ├── editors/   # TUI editors and related tools
+│   │   ├── editors/   # Heavy editor/tooling pkgs only (`packages.nix`; core editors live under `core/editors`)
 │   │   ├── encryption/ # GPG, password-store, etc.
 │   │   └── ...
 │   └── home.nix       # Main home manager entry point