fix: file in _instructions directory not set correctly.

[assertion]

For file (not directory) in _instructions, right now just history_truncated.md, the instructions is not set correctly:

and the {{ instructions.history_truncated }} in prompt template will not be rendered.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (main@a58f41a). Click here to learn what that means.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1602   +/-   ##
=======================================
  Coverage        ?   46.54%           
=======================================
  Files           ?       90           
  Lines           ?     3693           
  Branches        ?        0           
=======================================
  Hits            ?     1719           
  Misses          ?     1974           
  Partials        ?        0           

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[rbren]

Thanks for the fix!