home/develop/emacs: add yasnippet snippets and make snippet dirs configurable

This commit makes two significant changes to the Emacs configuration:

1. Refactors emacs.nix to use proper NixOS module structure:
   - Converts from simple configuration to a module with options/config
   - Adds `tb.emacs.extraSnippetDirs` option to allow per-machine snippet
     directories (defaults to ~/.emacs.d/snippets/)
   - Moves yasnippet configuration from custom.el to nix-generated elisp,
     enabling dynamic snippet directory configuration via Nix

2. Adds a comprehensive collection of yasnippet snippets for multiple modes:
   - C++ mode: 50 snippets for common patterns (class, namespace, operators,
     copy/move semantics, debug helpers, documentation, threading primitives)
   - CC mode (shared C/C++): for, switch, main, include guards, etc.
   - CMake mode: if/else, foreach, function, macro, project boilerplate
   - LaTeX mode: document structure, beamer frames, figures, lists
   - Perl mode: package boilerplate, shebang
   - Python/Shell/Nix/Protobuf/Java/CSS/Dockerfile modes: horizontal rules
     and common patterns
   - Text mode: modeline snippets for locale settings

The snippets use yasnippet's inheritance system via .yas-parents files,
so c++-mode inherits from cc-mode and text-mode, reducing duplication.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

home/develop/emacs/snippets/latex-mode/template

@@ -0,0 +1,73 @@
+# -*- mode: snippet; require-final-newline: nil -*-
+# name: template
+# key: template
+# binding: direct-keybinding
+# --
+% -*- mode: latex; mode: flyspell; ispell-local-dictionary: "en_US"; coding: utf-8 -*-
+
+\documentclass[a4paper,12pt,twoside]{scrartcl}
+
+\setlength\overfullrule{14pt} % this enables big black bars on overlong lines.
+
+\usepackage[utf8]{inputenc}
+\usepackage[T1]{fontenc}
+
+\usepackage{lmodern}
+\usepackage[english]{babel}
+
+\title{${1:title}}
+\author{${2:Timo Bingmann}}
+
+% small margin
+\usepackage[tmargin=20mm,bmargin=20mm,lmargin=20mm,rmargin=20mm]{geometry}
+
+% set default paragraph skip and indention
+\setlength\parskip{\smallskipamount}
+\setlength\parindent{0pt}
+
+% math support
+\usepackage{amsmath,amssymb,array,mathtools}
+
+% smaller enumerations
+\usepackage{enumitem}
+\setlist[enumerate]{topsep=\smallskipamount,parsep=0pt}
+\setlist[itemize]{topsep=\smallskipamount,parsep=0pt}
+\setlist[enumerate,1]{topsep=\smallskipamount,parsep=0pt,label=(\roman*)}
+
+% for graphics
+\usepackage{graphicx}
+
+% for nice graphics
+\usepackage{tikz}
+\usetikzlibrary{calc}
+
+% Hyperref
+\usepackage{xcolor,hyperref}
+
+\hypersetup{
+  colorlinks=true,
+  pdfborder={0 0 0},
+  bookmarksopen=true,
+  bookmarksopenlevel=1,
+  bookmarksnumbered=true,
+  linkcolor=blue!60!black,
+  %linkcolor=black,
+  urlcolor=blue!60!black,
+  filecolor=green!60!black,
+  citecolor=blue!60!black,
+  pdfpagemode=UseNone,
+  unicode=true,
+  setpagesize=false,
+  bookmarksdepth=subsection,
+  pdftitle={$1},
+  pdfauthor={$2},
+  pdfsubject={},
+}
+
+\begin{document}
+
+\maketitle
+
+$0
+
+\end{document}