fix: prevent stale pull request notification reports

[burmudar]

Generates notification reports from current PR state and validates that state before publishing a comment.

• Use go-github v92 for webhook types and live PR reads.
• Use exact base/head commits for the diff, base subscriptions, and report header.
• Prepare comment mutations before the final state check; retry a base change once and skip superseded, draft, or closed PRs.

Test plan

$ go test ./... -run '^TestGitHubNotificationsCurrentComparison$' -count=1
ok  github.com/sourcegraph/codenotify  5.960s

$ go test ./... -count=1
ok  github.com/sourcegraph/codenotify  5.841s

$ docker build --platform linux/amd64 --progress=plain -t codenotify:current-pr-check .
#13 naming to docker.io/library/codenotify:current-pr-check done
#13 DONE 0.5s

[sourcegraph-bot]

Codenotify: Notifying subscribers in CODENOTIFY files for diff 0c93fc6...e99471e.

Notify	File(s)
@nicksnyder	README.md go.mod go.sum main.go main_test.go