Add lost-in-conversation writing control bench

[yha9806]

Summary

• Adds a public-safe lost-in-conversation writing-control bench fixture comparing normal multi-turn editing, consolidated prompting, and thesis-control artifacts.
• Adds a structural bench checker and T73 regression guard.
• Links the bench from README and the product design doc.

Test Plan

• python3 scripts/audit-public-content.py --base-dir . --json
• git diff --check
• python3 scripts/check_lost_in_conversation_bench.py examples/lost-in-conversation-bench --json
• python3 .claude/skills/thesis-control/scripts/check_thesis_control.py examples/lost-in-conversation-bench/treatment --strict --json
• make test