CLI: no way to discover valid relationship endpoint types from `--help`

[Mearman]

Problem

sysprom update add-rel validates that relationship endpoint types are compatible (e.g. protocol → capability is invalid for implements), but there is no way to discover valid combinations from --help or any other CLI command.

The error messages are helpful after you get it wrong:

Invalid endpoint types for implements: protocol → capability.
Valid: [element, realisation, change, stage] → [capability, element, realisation, decision, change]

But ideally this information would be discoverable upfront.

Reproduction

sysprom update add-rel --help
# Shows: <from> <type> <to> with type choices — but no information about which node types are valid endpoints for each relationship type

sysprom update add-rel PROT1 implements CAP1
# Error with valid types listed

Suggestion

Either:

1. Add a sysprom query relationship-types (or similar) command that lists valid endpoint combinations per relationship type, or
2. Include a summary in sysprom update add-rel --help