fix: preserve read_file results during micro_compact

[stablegenius49]

Summary

• keep older read_file tool results intact during micro_compact so file contents remain available as reference material
• continue compacting older non-read_file tool outputs to placeholders
• add a regression test for the loop-inducing case and regenerate the lesson site extraction artifacts

Testing

• python3 -m unittest tests/test_s06_micro_compact.py -v
• python3 -m py_compile agents/s06_context_compact.py
• cd web && npm run extract
• cd web && npm run build

Closes #62

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