feat: warn on skipped malformed JSON lines in session logs

[dbroeglin]

This PR improves the robustness of events.jsonl parsing by adding a rich console warning when malformed JSON lines are encountered and skipped.

Previously, load_events() in sessionlog.py silently caught json.JSONDecodeErrors, which could leave researchers unaware if their session logs were truncated (e.g., from an abrupt agent exit or OOM kill). With this change, a warning message is emitted to stderr specifying the file name and the number of skipped lines.

Changes:

• Added a skipped_lines counter in load_events().
• Emits a warning via rich.console.Console(stderr=True) if any lines are skipped.
• Updated unit tests in test_sessionlog.py to capture and verify the stderr warnings.