Drop image subcommand from the CLI

[jserv]

The dispatcher existed solely to route to a single subcommand, forcing every invocation, doc, and test script to spell "kbox image" for no disambiguation value. Collapse the wrapper and let kbox [OPTIONS] -- COMMAND run directly. Also fix a latent issue where a positional command after -- would override -c, and add unit tests for the positional and -c+extras paths.

Change-Id: I3a927bb58163fc7528973e791b810ce6dc426fc2

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Summary by cubic

Simplifies the CLI by removing the image subcommand. You now run commands as kbox [OPTIONS] -- COMMAND, with cleaner parsing and clearer help.

• Refactors

  • Drop the image subcommand and dispatcher; options are parsed at the top level.
  • Fix parsing so -c CMD -- ARGS keeps extra args and isn’t overridden by a positional command.
  • Add -h help flag and update usage text.
  • Replace kbox_args with kbox_image_args; kbox_parse_args(argc, argv, struct kbox_image_args*).
  • Update docs, test scripts, and examples to use kbox ... instead of kbox image ....
  • Add unit tests for positional command parsing and -c with extra args.

• Migration

  • Replace all kbox image invocations with kbox.
  • Use -- to separate kbox options from the guest command.
  • Help is now kbox --help (or kbox -h).
  • If calling the parser from code, update to the new kbox_parse_args signature and types.

^{Written for commit 0000353. Summary will update on new commits.}

[cubic-dev-ai]

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