Support collapsing of sub-entities in output

[mindsocket]

Relates to #4

When outputting lists or diagrams, or syncing to another format (like Miro) it may be preferable to "collapse" rather than filter out sub-entities.

For example, instead of a list command outputting the exact node structure that was parsed in detail:

* My vision
  * My mission
    * My 1st goal
      * My 1st subgoal 
        * My 1st opportunity
      * My 2nd subgoal
    * My 2nd goal
      * My 2nd opportunity

... collapsing the mission and sub-goal layers could produce something like:

* My vision - My mission
  * My 1st goal - subgoals: My 1st subgoal, My 2nd subgoal 
    * My 1st opportunity
  * My 2nd goal
    * My 2nd opportunity

We'd need to come up with what kinds of collapsing need to be supported, ideally keep as simple as possible. I imagine 2 types: roll up entire type eg mission (to vision), and roll up sub-entities of type eg sub-goals (to top level goal)

For syncing to formats like miro, this collapsing creates a target system node tree that's more compact. Notably, it no longer adheres to node rules (goal has a collapsed vision-mission parent). We'll need to think about implications and solution design, considering:

• potential for 2-way sync - may not be possible or worth supporting sync back from a collapsed structure unless the target format can preserve that structure some other way
• we already have a somewhat similar concept in "embedded nodes" where in markdown space page can refer to a node that's embedded in a parent page