Fix return Thread node ID in get_review_comments response

[juidesai]

Problem

pull_request_read with method: get_review_comments never returns the thread
node ID (e.g. PRRT_kwDO...). The README documents using this ID with
resolve_thread, making that workflow completely broken.

Root Cause

convertToMinimalReviewThread() in pkg/github/minimal_types.go maps
reviewThreadNode → MinimalReviewThread, but MinimalReviewThread had no
ID field, so thread.ID was silently dropped.

Fix

Added ID string to MinimalReviewThread and populate it via
fmt.Sprintf("%v", thread.ID) in convertToMinimalReviewThread.

Result

get_review_comments now includes "id" on each thread object, enabling the
documented resolve_thread workflow end-to-end.