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/shells/config.nu

@@ -78,9 +78,9 @@ $env.config.recursion_limit = 50
 $env.config.edit_mode = "vi"
 
 # Command that will be used to edit the current line buffer with Ctrl+O.
-# If unset, uses $env.VISUAL and then $env.EDITOR
+# If unset, uses $env.VISUAL and then $env.EDITOR ($EDITOR is `hx` via session-env).
 #
-$env.config.buffer_editor = ["nvim", "--clean"]
+$env.config.buffer_editor = ["hx"]
 
 # cursor_shape_* (string)
 # -----------------------