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

tests/test_env_compat.py

@@ -0,0 +1,37 @@
+"""CHATGPT_API_KEY must keep working as an alias for OPENAI_API_KEY (backward
+compat carried over from the pre-SDK pageindex.utils; PR #272 review).
+
+The alias runs at import time in pageindex/__init__.py, so each case runs in a
+fresh subprocess with a controlled environment. cwd is a temp dir so load_dotenv
+can't pick up the repo's own .env and skew the result."""
+
+import os
+import subprocess
+import sys
+from pathlib import Path
+
+REPO = Path(__file__).resolve().parent.parent
+_PRINT_OPENAI = "import pageindex, os; print(os.environ.get('OPENAI_API_KEY', ''))"
+
+
+def _run(tmp_path, **overrides):
+    env = {k: v for k, v in os.environ.items()
+           if k not in ("OPENAI_API_KEY", "CHATGPT_API_KEY")}
+    env["PYTHONPATH"] = str(REPO)
+    env.update(overrides)
+    r = subprocess.run(
+        [sys.executable, "-c", _PRINT_OPENAI],
+        env=env, cwd=str(tmp_path), capture_output=True, text=True,
+    )
+    assert r.returncode == 0, r.stderr
+    return r.stdout.strip()
+
+
+def test_chatgpt_api_key_aliases_openai(tmp_path):
+    # Only CHATGPT_API_KEY set -> OPENAI_API_KEY gets filled from it.
+    assert _run(tmp_path, CHATGPT_API_KEY="sk-alias-123") == "sk-alias-123"
+
+
+def test_existing_openai_api_key_is_not_overwritten(tmp_path):
+    # Both set -> the real OPENAI_API_KEY wins; the alias must not clobber it.
+    assert _run(tmp_path, OPENAI_API_KEY="sk-real", CHATGPT_API_KEY="sk-alias") == "sk-real"