#51 [external:hu13] issue edit should accept --body-file (parity with create)

Filing from hu13 (external — do not implement without authorisation).

f0rge issue create takes --body-file; issue edit only takes inline --body. Editing a long issue body (e.g. updating a plan/rubric) from scripts requires shell-quoting the whole body (backticks execute in double quotes) — I had to shell out through python to pass it safely as argv.

Ask: add --body-file F to issue edit (and pr edit if it exists). No behaviour change otherwise.