Filter Guidelines service queries by content guideline type

[gziolo]

Summary

• Restrict Guidelines::fetch_guidelines() to the content term of the new wp_guideline_type taxonomy so a newer artifact guideline can't shadow the content singleton in prompt assembly.
• Fall back to the existing "newest wins" query when the taxonomy isn't registered (Gutenberg < 23.1), preserving backwards compatibility.
• Add Guidelines::TAXONOMY / Guidelines::TERM_CONTENT constants and three regression tests covering the content-vs-artifact preference, artifact-only sites, and the pre-taxonomy fallback path.

Fixes #529.

Test plan

• composer run-script format — clean
• composer run-script lint — clean
• composer run-script phpstan — clean for changed files (pre-existing unrelated Requirements.php warning)
• npm run test:php -- --filter Guidelines_Test — 31/31 pass
• npm run test:php -- --filter Abstract_Ability_Guidelines_Test — 9/9 pass

🤖 Generated with Claude Code

[github-actions]

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Co-authored-by: gziolo <gziolo@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>
Co-authored-by: henryperkins <htperkins@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.18%. Comparing base (514a7a7) to head (c541748).

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #593      +/-   ##
=============================================
+ Coverage      73.16%   73.18%   +0.02%     
- Complexity      1730     1731       +1     
=============================================
  Files             85       85              
  Lines           7467     7473       +6     
=============================================
+ Hits            5463     5469       +6     
  Misses          2004     2004              

Flag	Coverage Δ
unit	73.18% <100.00%> (+0.02%)	⬆️

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