feat: implement build backend and += env operator, add style guide

- Replace build stub with full phase execution (prepare/build/check/install)
  via /bin/sh with variable interpolation, config file generation, and
  env handling (hard-set, soft/?=, append/+=)

- Add += append operator for environment variables across DSL, parser,
  system config parser, build backend, and formatter

- Add STYLEGUIDE.md documenting all codebase conventions

- Replace EnvEntry bool soft with EnvMode enum (Set/Soft/Append)

- Add Plus token type to lexer for += parsing

src/dsl/lexer.cpp

@@ -42,6 +42,7 @@ std::string_view token_name(TokenType type) {
     case TokenType::Lbrace:    return "{";
     case TokenType::Rbrace:    return "}";
     case TokenType::Equals:    return "=";
+    case TokenType::Plus:      return "+";
     case TokenType::Lbracket:  return "[";
     case TokenType::Rbracket:  return "]";
     case TokenType::Comma:     return ",";
@@ -143,7 +144,7 @@ Token Lexer::next() {
             return {TokenType::Newline, "\n", token_start_line_, token_start_col_};
         }
         if (c == '"') { return scan_string(); }
-        if (c == '{' || c == '}' || c == '=' || c == '[' || c == ']' || c == ',') {
+        if (c == '{' || c == '}' || c == '=' || c == '+' || c == '[' || c == ']' || c == ',') {
             return scan_symbol();
         }
         return scan_ident();
@@ -155,7 +156,7 @@ Token Lexer::scan_ident() {
     while (pos_ < source_.size()) {
         char c = peek();
         if (std::isspace(static_cast<unsigned char>(c))) { break; }
-        if (c == '"' || c == '=' || c == '[' || c == ']' || c == ',') { break; }
+        if (c == '"' || c == '=' || c == '+' || c == '[' || c == ']' || c == ',') { break; }
         lexeme += advance();
     }
 
@@ -197,6 +198,7 @@ Token Lexer::scan_symbol() {
     case '{': return {TokenType::Lbrace,   "{", token_start_line_, token_start_col_};
     case '}': return {TokenType::Rbrace,   "}", token_start_line_, token_start_col_};
     case '=': return {TokenType::Equals,   "=", token_start_line_, token_start_col_};
+    case '+': return {TokenType::Plus,     "+", token_start_line_, token_start_col_};
     case '[': return {TokenType::Lbracket, "[", token_start_line_, token_start_col_};
     case ']': return {TokenType::Rbracket, "]", token_start_line_, token_start_col_};
     case ',': return {TokenType::Comma,    ",", token_start_line_, token_start_col_};