add instructions about TestEnv.jl to Sienna.md

[luke-kiernan]

No description provided.

[Copilot]

Pull request overview

Updates the Sienna agent/contribution guidance to document a TestEnv.jl-based test workflow alongside the existing --project=test commands, plus additional troubleshooting guidance.

Changes:

• Adds shell one-liners for running all tests or a specific test file using TestEnv.activate().
• Adds a note about avoiding environment-merge issues when using TestEnv.jl.
• Extends troubleshooting with an “Environment issues” section.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.86%. Comparing base (5f7f677) to head (83cfb3f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #586      +/-   ##
==========================================
- Coverage   78.88%   78.86%   -0.02%     
==========================================
  Files          75       75              
  Lines        6660     6649      -11     
==========================================
- Hits         5254     5244      -10     
+ Misses       1406     1405       -1     

Flag	Coverage Δ
unittests	78.86% <ø> (-0.02%)	⬇️

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see 6 files with indirect coverage changes

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[luke-kiernan]

Turns out using TestEnv; TestEnv.activate(); push!(ARGS, "test_file_name"); include("test/runtests.jl") doesn't work. Will revise