feat(markdown-config): register markdown as an org src-block language

`#+begin_src markdown ... #+end_src' blocks rendered and exported
fine but `org-lint' warned on every one of them ("Unknown source
block language: 'markdown'"), and `C-c '' inside the block fell
back to `fundamental-mode' instead of opening it in
`markdown-mode' for editing.

Add a `with-eval-after-load 'org' form that pushes
`("markdown" . markdown)' onto `org-src-lang-modes'.  New ERT test
in `tests/test-markdown-config.el' asserts the entry resolves to
`markdown' after `(require 'markdown-config)'.

Surfaced while clearing `org-lint' on `todo.org' from 55 issues
down to 1 -- the last one was this warning on a Linear ticket-body
draft that was genuinely markdown.  Registering the language is
the right fix; relabeling the block as `text' or `example' would
lose accuracy.

0 files changed, 0 insertions, 0 deletions