Add discussions

Discussions are served from GitHub's older server-rendered HTML, with no
embeddedData JSON to read, so the scraper walks the DOM the way the pull
request and wiki code does. A discussion's timeline opens with its own
post and continues with its comments, each carrying its replies nested
inside it, which is where the recursion in _discussionComment.eta comes
from. Categories, upvotes, and the answer a Q&A discussion is marked with
all come along.

Only what GitHub renders up front is shown: neither the next page of
discussions nor the replies it hides behind a "Show N previous replies"
button are fetched, so a comment says how many replies it is missing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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