feat: preserve @startmindmap title text in conversion

[larkly]

Description

PlantUML supports an optional title on the opening marker:

@startmindmap My Map Title
* Root
@endmindmap

The converter currently recognises the line as the start marker but silently discards everything after @startmindmap. No error is raised and no title is preserved in the output .mm file.

Expected behaviour

If a title is present it should either be round-tripped (e.g. stored as a Freeplane map attribute or a dedicated node) or, at minimum, documented as an unsupported feature and warned about.