refactor: extract IO utilities to _output.py

[asmacdo]

Summary

• Extract I/O utilities to new _output.py module:
  • TailPipe class
  • prepare_outputs function
  • safe_close_files function
  • remove_files function
• Update test imports and patches to use new module location
• Remove unused Iterable import from duct_main.py

Part of #372

Test plan

• All 346 tests pass
• Lint passes
• Typing passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 92.40506% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.72%. Comparing base (9b96e1b) to head (57c12d0).
⚠️ Report is 3 commits behind head on duct-main-breakup.

Files with missing lines	Patch %	Lines
src/con_duct/_output.py	92.30%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           duct-main-breakup     #382      +/-   ##
=====================================================
+ Coverage              91.48%   91.72%   +0.24%     
=====================================================
  Files                     13       14       +1     
  Lines                   1092     1100       +8     
  Branches                 138      138              
=====================================================
+ Hits                     999     1009      +10     
+ Misses                    70       69       -1     
+ Partials                  23       22       -1     

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