refactor: extract SigIntHandler to _signals.py

[asmacdo]

Summary

• Extract SigIntHandler class from duct_main.py to new _signals.py module
• Remove unused FrameType 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 50.00000% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.42%. Comparing base (64e72c9) to head (1786925).
⚠️ Report is 3 commits behind head on duct-main-breakup.

Files with missing lines	Patch %	Lines
src/con_duct/_signals.py	48.00%	13 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           duct-main-breakup     #380      +/-   ##
=====================================================
- Coverage              91.55%   91.42%   -0.13%     
=====================================================
  Files                     11       12       +1     
  Lines                   1077     1084       +7     
  Branches                 138      138              
=====================================================
+ Hits                     986      991       +5     
- Misses                    69       70       +1     
- Partials                  22       23       +1     

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