Remove deepcopy and reduce memory allocations to prevent Lambda OOM

[hiroshinishio]

Summary

• Removed deepcopy(messages) in remove_outdated_apply_diff_to_file_attempts_and_results - this was duplicating the entire conversation history (all messages, diffs, tool results) on every Claude API call. At 38+ rounds with growing history, this alone could account for hundreds of MB.
• Changed [system_msg] + list(messages) to [system_msg, *messages] in chat_with_claude to avoid an unnecessary intermediate list copy.
• Added gc.collect() with RSS logging after each agent loop iteration in all 3 handlers to force garbage collection of temporary objects between rounds.
• Extracted get_rss_mb() into a shared utility (utils/memory/get_rss_mb.py) to deduplicate RSS measurement logic between is_lambda_oom_approaching and the new gc_collect_and_log utility.