`jira` provider (`list` + `comments`)

[abdotop]

Depends on: Issue 1, Issue 2

Implement the jira Provider:

• list(scope) — query jira/issue scoped by scope.jiraProjectKey (see
  api/STORE_API.md for the jira/issue shape), map to WorkItem[]
  (key straight into the ticket key, Assignee into assigneeRefs)
• comments(item) — Description field as the first entry, Comment
  field as the rest (see §5)

Acceptance criteria

• list() returns real jira/issue records for the pilot project's
  Jira key, correctly shaped as WorkItem[]
• comments() returns the description + comments for one real issue
• Confirms/documents the actual Assignee field shape
  (emailAddress/accountId) against real data