github provider (list + comments) #161

Depends on: Issue 1, Issue 2

Implement the github Provider:

• list(scope) — query github/issue scoped by scope.repositoryUrl,
  extract the key from the enforced title prefix, map to WorkItem[]
• comments(item) — issue/PR body as the first entry, then github/comment
  • github/review bodies scoped by .issue == item.externalId
• assigneeRefs/reviewerRefs — reviewerRefs from github/review
  authors; assigneeRefs needs verifying against real data first (see §3,
  §7 — the cached schema doesn't obviously expose a distinct assignees
  field, participants may be the closest proxy)

Acceptance criteria

• list() returns real PRs/issues for the pilot repo, keyed correctly
  when the title carries the {KEY} prefix
• comments() merges body + github/comment + github/review in
  chronological order
• Resolves the assignee-field question from §3/§7 with a concrete
  answer (which field, or "not available")