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/tools/format.cpp

@@ -18,7 +18,13 @@ static void write_env(std::ostream& os, int d,
     if (entries.empty()) { return; }
     os << Indent(d) << "env {\n";
     for (auto& e : entries) {
-        os << Indent(d + 1) << e.key << (e.soft ? " ?= " : " = ")
+        const char* op;
+        switch (e.mode) {
+        case dsl::EnvMode::Soft:   op = " ?= "; break;
+        case dsl::EnvMode::Append: op = " += "; break;
+        default:                   op = " = ";  break;
+        }
+        os << Indent(d + 1) << e.key << op
            << '"' << e.value << "\"\n";
     }
     os << Indent(d) << "}\n";