v0.5.0

Add markdown-to-ADF prose, label add/remove, and attachments

Five new operations on both surfaces: add-labels, remove-labels,
attach, delete-attachment, and markdown-to-adf. Comment, create, and
update gain --description-format markdown, which converts the body to
ADF and posts via api/3; plain text on api/2 stays the default.

The format value parses strictly (a typo errors instead of silently
posting raw markdown), and a pipe-opened line without a closing pipe
no longer hangs the ADF table parser.

Assisted-by: Claude Fable 5 <noreply@anthropic.com>