fix: tests failed when git is not installed

[ycexiao]

• fix: skip agentify tests when git or internet is unavailable
• feat: ensure git and internet connection
• fix: use diffpy-app instead of diffpy.app

What problem does this PR address?

The Conda-forge build failed: https://dev.azure.com/conda-forge/84710dde-1620-425b-80d0-4cf5baca359d/_apis/build/builds/1524931/logs/29

1. Ensure internet connection and git is installed before running agentry to fetch skills.
2. diffpy.app is not working in macOS system because of the suffix .app. Use diffpy-app instead.

What should the reviewer(s) do?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.14%. Comparing base (5512cb8) to head (974f317).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #28 +/- ##
==========================================
+ Coverage 94.11% 94.14% +0.03% 
==========================================
Files 5 5 Lines 187 188 +1 ==========================================
+ Hits 176 177 +1 
Misses 11 11 

Files with missing lines	Coverage Δ
tests/test_agentify.py	100.00% <100.00%> (ø)

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