Split the pagx convert command into import and export

[OnionsYu]

Summary

• Split the single convert command into separate import and export subcommands for clearer CLI semantics
• export converts PAGX to other formats (SVG), import converts other formats (SVG) to PAGX
• Updated tests to cover both new commands independently

Test plan

• Existing convert tests migrated to import/export tests
• Verify pagx export --input file.pagx produces SVG output
• Verify pagx import --input file.svg produces PAGX output
• Verify error handling for missing input and unsupported formats

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.28713% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.59%. Comparing base (0538ea0) to head (f657098).

Files with missing lines	Patch %	Lines
src/cli/CommandExport.cpp	91.20%	2 Missing and 6 partials ⚠️
src/cli/CommandImport.cpp	91.76%	2 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3347      +/-   ##
==========================================
+ Coverage   75.19%   75.59%   +0.39%     
==========================================
  Files         509      510       +1     
  Lines       35211    35431     +220     
  Branches    11270    11281      +11     
==========================================
+ Hits        26478    26785     +307     
+ Misses       6428     6354      -74     
+ Partials     2305     2292      -13     

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