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

include/kappa/build/build.hpp

@@ -13,9 +13,8 @@ struct BuildResult {
 };
 
 // Build a single package step into work_dir.
-//
-// NOTE: compile-only stub — the real build backend was never committed.
-// Always fails so the rebuild pipeline never records a fake install.
+// Runs prepare, build, check, and install phases in order,
+// with full variable interpolation and config file generation.
 BuildResult build(const resolve::BuildStep& step,
                   const std::string& work_dir,
                   int jobs);

include/kappa/dsl/ast.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <cstdint>
 #include <string>
 #include <unordered_map>
 #include <unordered_set>
@@ -38,10 +39,12 @@ struct Patch {
     int level = 1;
 };
 
+enum class EnvMode : std::uint8_t { Set, Soft, Append };
+
 struct EnvEntry {
     std::string key;
     std::string value;
-    bool soft = false;
+    EnvMode mode = EnvMode::Set;
 };
 
 struct NamedService {

include/kappa/dsl/token.hpp

@@ -13,6 +13,7 @@ enum class TokenType {
     Lbrace,     // {
     Rbrace,     // }
     Equals,     // =
+    Plus,       // +
     Lbracket,   // [
     Rbracket,   // ]
     Comma,      // ,