feat(github): per-repo commit enumeration for full history backfill

Adds a new github-repo EventSource that enumerates all repos via
/user/repos and walks each repo's /commits?author= endpoint, which
has no 1000-result cap unlike the Search API. Events use the same
github-commit:{sha} ID scheme as github_search for dedup. Per-repo
poller state enables full backfill on first run, page-1-only on
subsequent polls. Weekly poll interval by default.

Closes #1

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

script/hg-ingest.sh

@@ -19,6 +19,7 @@ WORK_DIR="${HG_WORK_DIR:-$HOME/hg}"
 
 # Repos to clone (groups are expanded inline)
 REPOS=(
+  mozilla-central
   integration/mozilla-inbound
   integration/autoland
   integration/fx-team