Track issues with git-bug

Issues live in git-bug, inside the Git repository, so they work offline
and travel with the code through git bug push and pull. They are not
files in the checkout, so they stay out of diffs, formatters, and the
rule that files describe only the current state.

The development shell supplies git-bug. The issues skill tells agents
how to read and write issues, and warns that the -F option drops every
line that starts with "#". Commit messages now refer to an issue with an
Issue trailer that holds its git-bug ID, because there are no issue URLs
to link.

git-bug has no links between issues, so relations are Parent and
Depends-on trailers at the end of the body, as in a commit message. git
interpret-trailers can read them, and a search for an issue ID finds the
issues that refer to it.