[fxadmin] json manipulation automation

Automate the manual JSON-editing step of the reconfiguration flow (decode → hand-edit → compute-update) by adding fxadmin modify commands that apply the most common configuration changes: node endpoints, node certificates, CA/TLS-CA cert lists, and adding/removing organizations and parties - directly to a config block, letting multiple changes accumulate into a single config update while the manual path remains supported.

Start at the fxadmin reconfiguration flow and trace the existing decode → hand-edit → compute-update path. Define how modify commands apply node endpoints, certificates, CA/TLS-CA certificate lists, and organization or party changes to one config block, while preserving the manual path; done means multiple changes accumulate into a single config update.

Written by the indexing model from the issue text.