feat(cli): implement exit delete

[varex83agent]

Closes #406.

Ports Charon's cmd/exit.go parent command and cmd/exit_delete.go as pluto exit delete, which removes this operator's partially signed exit message(s) from the remote Obol API. Flags, defaults and help texts match charon's bindExitFlags subset (--publish-address, --private-key-file, --lock-file, --validator-public-key, --all, --publish-timeout and the five --testnet-* flags), including the two pre-run flag checks with charon's verbatim error messages.

The share index comes from the cluster lock and the loaded identity key (charon's keystore.ShareIdxForCluster), and --all warns-and-continues on a missing exit rather than aborting. Charon's TestExitDeleteCLI is ported directly; testRunDeleteExitFullFlow is ported against a wiremock Obol API (charon's version depends on exit sign, which is issue #409). Only the minimal parent exit command needed to host delete was added.

Co-Authored-By: Bohdan Ohorodnii 35969035+varex83@users.noreply.github.com