Version-aware galgebra detection and CI modernization

[utensil]

Summary

Supersedes #22. Implements a coherent Python dependency management strategy:

• deps/build.jl: Detects existing galgebra in the user's Python environment, checks version against range [0.6.0, 0.7.0), and only installs a controlled version when galgebra is missing or out of range
• src/GAlgebra.jl: Replaces pyimport_conda with plain pyimport (no more Conda fallback), adds GALGEBRA_DEBUG env var for diagnostics output
• CI: Explicit pip install galgebra==0.6.0 with PYTHON env var, bumped Actions (checkout v4, setup-python v5, setup-julia v2, codecov v4)
• README: Documents Python dependency behavior, custom Python environments, and diagnostics

Design decisions

• Version range (not exact pin): allows users to use any compatible galgebra in [0.6.0, 0.7.0)
• Respects user's Python: works with system Python, virtualenvs, or any Python that PyCall is configured to use
• GALGEBRA_DEBUG=1: prints galgebra version and Python path at load time for troubleshooting

Test plan

• CI passes on Julia 1.10 and 1.11 with Python 3.12
• Verify GALGEBRA_DEBUG=1 output shows version and Python path
• Verify build script detects pre-installed galgebra and skips install

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.12%. Comparing base (5b9a101) to head (724cbb4).

Files with missing lines	Patch %	Lines
src/GAlgebra.jl	70.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
- Coverage   80.43%   78.12%   -2.31%     
==========================================
  Files           4        4              
  Lines          92       96       +4     
==========================================
+ Hits           74       75       +1     
- Misses         18       21       +3     

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