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/README.md

@@ -10,9 +10,7 @@ systems.
 - **core/**: Essential cross-platform configurations
   - **core.nix**: Minimal home-manager configuration
   - **shells/**: Shell configurations (bash, zsh, fish, nu)
-  - **editors/**: Text editor configurations
-    - **neovim/**: Neovim with custom plugins and settings
-    - **helix/**: Helix editor configuration
+  - **editors/**: Helix + Neovim (Home Manager) and usage docs (`README.md` per editor)
   - **btop.nix**: System monitoring tools
   - **git.nix**: Git configuration and aliases
   - **npm.nix**: Node.js package management
@@ -39,7 +37,7 @@ systems.
   - **cloud/**: Cloud development tools (Terraform, etc.)
   - **container.nix**: Container tools (Docker, Podman)
   - **dev-tools.nix**: Terminal-based development tools
-  - **editors/**: Terminal editor configurations
+  - **editors/**: Extra terminal editor Nix (see `core/editors/` for docs and baseline enables)
   - **encryption/**: Encryption and security tools
   - **gpg/**: GPG key management
   - **password-store/**: Password management with pass