Add Yardang Sphinx extension

[timkpaine]

Description

Add a no-op yardang.sphinx extension and load it from generated Sphinx configuration. This establishes the extension namespace for later Yardang-owned integrations without moving existing hooks or changing llms generation yet.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup
• CI / build configuration
• Other (describe below)

Checklist

• Linting passes (make lint)
• Tests pass (make test)
• New tests added for new functionality
• Documentation updated (not applicable for this no-op scaffold)
• Changelog / version bump (not applicable)

[github-actions]

Test Results

83 tests  +2   83 ✅ +2   20s ⏱️ +6s
 1 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit 27c3335. ± Comparison against base commit 0803d84.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.05%. Comparing base (0803d84) to head (27c3335).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #133      +/-   ##
==========================================
+ Coverage   86.93%   87.05%   +0.11%     
==========================================
  Files          12       14       +2     
  Lines        1745     1761      +16     
  Branches      183      183              
==========================================
+ Hits         1517     1533      +16     
  Misses        177      177              
  Partials       51       51              

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