[WIP] Fix truncate behavior for files opened in write mode

[Copilot]

• Inspect stream mode flags and existing open-mode coverage
• Add write-mode truncation flag and focused regression coverage
• Run targeted and complete engine tests; scan changed files for secrets
• Commit review and CodeQL verification

• Fixes 🐛 Truncate existing files opened in write mode #89

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff           @@
##             main      #98   +/-   ##
=======================================
  Coverage   98.33%   98.33%           
=======================================
  Files          25       25           
  Lines        7194     7194           
=======================================
  Hits         7074     7074           
  Misses         88       88           
  Partials       32       32           

Files with missing lines	Coverage Δ
engine/stream.go	100.00% <ø> (ø)

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