Split main.cpp into cli_args, commands, and self_test

main.cpp had grown to ~910 lines holding CLI parsing, every command
implementation, and the -t/--test handler all in one file. Split it:

- cli_args.{h,cpp}: getopt_long parsing and all post-loop validation
  (parse_args()), producing a ParsedArgs the rest of the program consumes.
- commands.{h,cpp}: every command implementation plus dispatch_command(),
  an exhaustive switch over Mode with no default -- so -Wswitch (this
  project's warning_level=3) now catches a future Mode value added without
  a matching dispatch case, instead of silently falling through. Confirmed
  by temporarily adding an unhandled enumerator and observing the warning.
- self_test.{h,cpp}: -t/--test's own file, ahead of real tests landing here.

Also fixes Mode::mount, which previously had no explicit dispatch check at
all -- it ran only because it was whatever fell off the end of main()'s
if/else chain when nothing else matched. It's now mount_command(), a real
case in dispatch_command() like every other mode.

main.cpp itself shrinks to ~30 lines: load config, apply its log level,
parse_args(), dispatch_command().

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gv3s5jckJKzh6JkMoi2Akz

meson.build

@@ -17,7 +17,8 @@ conf_data.set10('ENABLE_TESTS', get_option('enable_tests'))
 configure_file(output : 'config.h', configuration : conf_data)
 
 slocker_lite = executable('slocker-lite',
-    ['src/main.cpp', 'src/process.cpp', 'src/oci_image.cpp', 'src/containers_storage.cpp',
+    ['src/main.cpp', 'src/cli_args.cpp', 'src/commands.cpp', 'src/self_test.cpp',
+     'src/process.cpp', 'src/oci_image.cpp', 'src/containers_storage.cpp',
      'src/bwrap.cpp', 'src/user_spec.cpp', 'src/config_file.cpp', 'src/volume_mount.cpp',
      'src/pid_file.cpp', 'src/exec_session.cpp', 'src/env_spec.cpp', 'src/daemonize.cpp'],
     include_directories : include_directories('.'),