Add structured JSON output to logout command

[AndreyVMarkelov]

Summary

• Adds --output=json support to logout with logged_out and already_logged_out statuses, reporting removed_saved_credentials and remote_token_revoked.
• Rejects logout when DBXCLI_ACCESS_TOKEN is set with a new env_token_still_active error code.
• Emits a token_revoke_failed warning when local credentials are removed but remote revocation fails.
• Iterates tokens deterministically (sorted domains and token types) for reproducible output.
• Adds long description to the logout command and updates generated docs.

Test plan

• go test ./... -count=1 passes (7 new logout tests + contract audits)
• golangci-lint run ./... clean
• go build succeeds
• Generated docs/commands/dbxcli_logout.md matches go run ./tools/gen-docs output
• CI passes