feat: add factory handoff CLI command for cross-session resume

[lukeinglis]

Summary

• New factory/handoff.py module with generate_handoff() that reads 9 data sources and synthesizes a structured markdown brief to stdout
• cmd_handoff added to CLI following the cmd_export pattern
• 4 tests covering empty, populated, partial data, and CLI wiring

Closes #352

[codecov]

Codecov Report

❌ Patch coverage is 78.40376% with 46 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.11%. Comparing base (8097461) to head (d619b2c).

Files with missing lines	Patch %	Lines
factory/handoff.py	80.00%	41 Missing ⚠️
factory/cli.py	37.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #373      +/-   ##
==========================================
- Coverage   87.31%   87.11%   -0.20%     
==========================================
  Files          61       62       +1     
  Lines        9339     9552     +213     
==========================================
+ Hits         8154     8321     +167     
- Misses       1185     1231      +46     

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