fix: exact md page selection + restore CHATGPT_API_KEY alias

Address PR #272 review:

- get_md_page_content / retrieve._get_md_page_content returned every node
  whose line_num fell in [min(pages), max(pages)], so a non-contiguous spec
  like "5,100" over-fetched everything in between. Match the exact requested
  line numbers instead, mirroring the PDF path. (Same bug as #280.)
- Restore the CHATGPT_API_KEY -> OPENAI_API_KEY backward-compat alias dropped
  when pageindex/utils.py became a re-export shim; users with only
  CHATGPT_API_KEY set would otherwise fail auth after upgrading. It now runs
  in __init__.py right after load_dotenv.

Adds regression tests for both.

Claude-Session: https://claude.ai/code/session_01Kx5DgKbhK1N8autqXH8SmS

pageindex/retrieve.py

@@ -56,16 +56,19 @@ def _get_pdf_page_content(doc_info: dict, page_nums: list[int]) -> list[dict]:
 def _get_md_page_content(doc_info: dict, page_nums: list[int]) -> list[dict]:
     """
     For Markdown documents, 'pages' are line numbers.
-    Find nodes whose line_num falls within [min(page_nums), max(page_nums)] and return their text.
+    Return only the nodes whose line_num is one of ``page_nums`` (exact match),
+    not the whole [min(page_nums), max(page_nums)] range.
     """
-    min_line, max_line = min(page_nums), max(page_nums)
+    if not page_nums:
+        return []
+    wanted = set(page_nums)
     results = []
     seen = set()
 
     def _traverse(nodes):
         for node in nodes:
             ln = node.get('line_num')
-            if ln and min_line <= ln <= max_line and ln not in seen:
+            if ln in wanted and ln not in seen:
                 seen.add(ln)
                 results.append({'page': ln, 'content': node.get('text', '')})
             if node.get('nodes'):