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/cmake-mode/.yas-parents

@@ -0,0 +1 @@
+text-mode

home/develop/emacs/snippets/cmake-mode/cmake_minimum_required

@@ -0,0 +1,5 @@
+# -*- mode: snippet -*-
+# name: cmake_minimum_required
+# key: min
+# --
+cmake_minimum_required(VERSION ${1:2.6})

home/develop/emacs/snippets/cmake-mode/foreach

@@ -0,0 +1,7 @@
+# -*- mode: snippet -*-
+# name: foreach
+# key: for
+# --
+foreach(${1:item} \${${2:array}})
+        $0
+endforeach()

home/develop/emacs/snippets/cmake-mode/function

@@ -0,0 +1,7 @@
+# -*- mode: snippet -*-
+# name: function
+# key: fun
+# --
+function (${1:name})
+         $0
+endfunction()

home/develop/emacs/snippets/cmake-mode/hr

@@ -0,0 +1,6 @@
+# -*- mode: snippet; require-final-newline: nil -*-
+# name: hr
+# key: hr
+# binding: direct-keybinding
+# --
+`(make-string (- 80 (current-column)) ?#)`

home/develop/emacs/snippets/cmake-mode/if

@@ -0,0 +1,7 @@
+# -*- mode: snippet -*-
+# name: if
+# key: if
+# --
+if(${1:cond})
+   $0
+endif()

home/develop/emacs/snippets/cmake-mode/ifelse

@@ -0,0 +1,9 @@
+# -*- mode: snippet -*-
+# name: ifelse
+# key: if
+# --
+if(${1:cond})
+        $2
+else(${3:cond})
+        $0
+endif()

home/develop/emacs/snippets/cmake-mode/include

@@ -0,0 +1,5 @@
+# type: command
+# key: inc
+# name: include
+# --
+include ($0)

home/develop/emacs/snippets/cmake-mode/macro

@@ -0,0 +1,7 @@
+# -*- mode: snippet -*-
+# name: macro
+# key: macro
+# --
+macro(${1:name}${2: args})
+
+endmacro()

home/develop/emacs/snippets/cmake-mode/message

@@ -0,0 +1,5 @@
+# -*- mode: snippet -*-
+# name: message
+# key: msg
+# --
+message(${1:STATUS }"$0")

home/develop/emacs/snippets/cmake-mode/option

@@ -0,0 +1,5 @@
+# -*- mode: snippet -*-
+# name: option
+# key: opt
+# --
+option (${1:OPT} "${2:docstring}" ${3:value})

home/develop/emacs/snippets/cmake-mode/project

@@ -0,0 +1,5 @@
+# -*- mode: snippet -*-
+# name: project
+# key: proj
+# --
+project ($0)

home/develop/emacs/snippets/cmake-mode/set

@@ -0,0 +1,5 @@
+# -*- mode: snippet -*-
+# name: set
+# key: set
+# --
+set(${1:var} ${2:value})