Introduce the pkg/cmd/discussion/ package with:
- DiscussionClient interface and domain types (client/)
- Generated mock via moq (client/)
- Factory function for lazy client creation (shared/)
- JSON field definitions for --json output (shared/)
- Root 'discussion' command registered in the core group
The interface defines all planned operations (list, search, get, create,
update, close, reopen, comment, lock, unlock, mark-answer, unmark-answer)
with stub implementations that will be replaced as each subcommand is
added in subsequent PRs.
Domain types are intentionally separate from API types per review guidance.
No JSON struct tags are used; serialization is handled by ExportData methods.
Refs: cli/cli#12810, github/gh-cli-and-desktop#115
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
