feat(twitter_threads): full-thread extraction (conversations) + front-matter from target + corpus dedupe

fetch_thread_from_url now passes -o conversations=true (full threads, not single tweets), sorts posts chronologically, and sets root_post_id to the URL's target tweet. render_markdown builds front-matter from the target (root_post_id) post, not the earliest conversation tweet (fixes wrong @handle). Added dedupe_corpus.py: keeps deepest thread per conversation, deletes duplicates, keeps+marks unique branches (branch_of) + writes threads_index.json. 33 tests pass.

scripts/twitter_threads/render_markdown.py

@@ -19,8 +19,14 @@ def _format_view_count(value: int | None) -> str:
   return "null"
  return str(value)
 
+def _root_post(thread: ThreadData) -> PostData:
+ for p in thread.posts:
+  if p.post_id == thread.root_post_id:
+   return p
+ return thread.posts[0]
+
 def _render_yaml(thread: ThreadData, title: str) -> list[str]:
- root = thread.posts[0]
+ root = _root_post(thread)
  return [
   "---",
   f'title: "{title}"',
@@ -59,7 +65,7 @@ def _render_post(post: PostData, n: int, id_to_index: dict[str, int], media_name
 def render_markdown(thread: ThreadData, media_names: dict[str, list[str]], output: Path) -> Result[Path]:
  lines: list[str] = []
  id_to_index = _build_id_to_index(thread)
- root = thread.posts[0]
+ root = _root_post(thread)
  first_line = root.text.splitlines()[0] if root.text.splitlines() else ""
  title = first_line[:80]
  lines.extend(_render_yaml(thread, title))