Clarify issue comment semantics and strengthen report_progress guidance (#292)

- Add `comment_type: "issue"` to `IssueCommentCreatedEvent` interface and
  dispatch sites so agents can distinguish issue comments from PR review
  comments
- Add dedicated "Progress reporting" section to system prompt making
  `report_progress` the mandatory tool for sharing results
- Update `reply_to_review_comment` description to clarify it only works
  for inline review comments on PR diffs, not issue comments

Co-authored-by: pullfrog[bot] <226033991+pullfrog[bot]@users.noreply.github.com>

external.ts

@@ -169,6 +169,8 @@ interface IssuesLabeledEvent extends BasePayloadEvent {
 interface IssueCommentCreatedEvent extends BasePayloadEvent {
   trigger: "issue_comment_created";
   comment_id: number;
+  /** distinguishes this from PR review comments (which use pull_request_review_comment_created) */
+  comment_type: "issue";
   /** comment body is the primary content (null if already in prompt) */
   body: string | null;
   issue_number: number;