A closed range like [23..28] wrongly implies guidance stops applying after
BC28, so a reviewer targeting BC29+ would not match the file. Introduce an
open-ended shorthand [N..] meaning ''version N and every later version''.
- validate_frontmatter.py: RANGE_SHORTHAND allows an optional upper bound;
expand_bc_version returns the normalized string ''N..'' for open-ended.
- read.md: document the fourth bc-version form and its matching rule
(matches target >= N; not enumerable).
- write.md: prefer [N..] over a closed range for a feature introduced in N
and not expected to be removed.
- Apply [23..] to the actionable-errors article (actionable errors shipped
in BC23 and are not version-bounded above).
- README: mention [N..] in the frontmatter example.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
