CLI: no command to add or manage `external_references`

[Mearman]

Problem

There is no CLI command to add, list, update, or remove external_references on the document or on individual nodes. The only way to add them is by editing the JSON directly.

Reproduction

# No such command exists:
sysprom update add-ref --node DEC1 --role evidence --identifier "docs/adr/0001.md"

Use case

External references are a core part of the SysProM model — they link nodes to source documents, ADRs, standards, and other external evidence. In practice, building a SysProM document via the CLI means you must drop to manual JSON editing for all external references, which undermines the value of having a CLI.

Suggestion

Add commands such as:

sysprom update add-ref <node-id> --role <role> --identifier <identifier> [--description <desc>]
sysprom update remove-ref <node-id> --identifier <identifier>

Or a document-level variant:

sysprom update add-ref --node-id <id> --role <role> --identifier <identifier>

The role choices should match the schema: input, output, context, evidence, source, standard, prior_art.