Expand language evolution notes for contributors

[DKFuH]

Why

OrgScript now needs clearer guidance for what belongs in the language core and how future changes should be evaluated.

Scope

• define what is considered a core language change
• define what should stay in tooling or exporters
• document how breaking changes are handled
• explain how spec evolution and CLI evolution should be coordinated

Definition Of Done

• contributors can understand how language changes are evaluated

• breaking changes are treated explicitly instead of implicitly

• the governance story is clearer than today

• v0.5.0

• documentation

• governance

• existing governance notes

• canonical language spec

Keep the process lightweight. This should guide decisions, not slow them down.