feat: portability, correctness, and quality improvements for v0.2

Portability (Linux distro-agnostic):
- Remove hardcoded Clang compiler enforcement; GCC now builds
- Add find_package(Threads REQUIRED) for older glibc
- Add cmake install() target
- FHS 3.0 default root: /kappa -> /usr/local/kappa
- fs::path operator/ for all init/bootloader paths (fixes prefix fragility)
- Multi-distro zoneinfo search (FHS, NixOS, Guix, alt)
- Portable tar extraction (drop GNU-only --no-same-permissions)
- Runit enable/disable commands now prefix-aware
- --root CLI flag before/after subcommand, lazy directory creation
- Shebang constants de-duplicated to types.hpp

Correctness (race conditions, UB, corruption):
- Fix CWD race in scheduler: per-child chdir() instead of process-global
- Fix UB const_cast in exec_cmd/exec_capture: mutable argv buffers
- Fix non-atomic installed DB writes: tmp+rename pattern
- Fix read_file() no longer calls exit(1), throws instead
- Fix silent catch(...) parse errors now print diagnostics
- Fix rebuild false positives with config_hash change detection
- Fix s6 disable_cmd copy-paste bug (was identical to enable)
- Fix runit enable_cmd incomplete, disable_cmd wrong target
- Fix dinit env vars: functional env-file + companion .env

Quality:
- Add -Wall -Wextra -Wpedantic to CMake, fix 2 pre-existing warnings
- Move parse_int from error.hpp to parse_util.hpp
- Fix hash verification guard checks all three hash types
- Check patch return code in fetch.cpp
- Add explicit system_dir creation in ensure_directories()
- Add resolve to needs_dirs for build_registry() consistency
- Update stale /kappa path references in examples
- Remove inaccurate -Werror claim in CONTRIBUTING.md
- Add build-gcc/ and agent dirs to .gitignore
- Suppress clang-tidy portability-avoid-pragma-once
- Fix .gitignore /kappa pattern (was matching include/kappa/)
- Delete stale vcpkg_installed/ directory

54/54 tests pass. Builds on Clang and GCC with 0 warnings.

CMakeLists.txt

@@ -1,11 +1,9 @@
 cmake_minimum_required(VERSION 3.20)
 
-# Enforce Clang
-set(CMAKE_C_COMPILER clang)
-set(CMAKE_CXX_COMPILER clang++)
-
 project(kappa VERSION 0.1.0 LANGUAGES CXX)
 
+find_package(Threads REQUIRED)
+
 set(CMAKE_CXX_STANDARD 23)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
@@ -53,3 +51,7 @@ add_executable(kappa
   src/system/activate.cpp
 )
 target_include_directories(kappa PRIVATE include)
+target_link_libraries(kappa PRIVATE Threads::Threads)
+target_compile_options(kappa PRIVATE -Wall -Wextra -Wpedantic)
+
+install(TARGETS kappa RUNTIME DESTINATION bin)