JSON Schemas: add Taxonomy schema

[priethor]

Part of #162

What

Add JSON schema for validating SCF taxonomy definitions.

Why

To enable later validation of taxonomy JSON exports/imports before processing, including in upcoming Taxonomy-related abilities.

How

• Created schemas/taxonomy.schema.json following the Post Type schema pattern
• Added PHPUnit tests
• Added real SCF export fixtures and Perplexity-generated invalid example

Testing Instructions

1. Run vendor/bin/phpunit tests/php/TaxonomySchemaTest.php and verify schema rejects invalid taxonomies (wrong prefix, reserved terms, bad characters)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 7.96%. Comparing base (4be2455) to head (99b47f0).
⚠️ Report is 1 commits behind head on trunk.

Additional details and impacted files

@@           Coverage Diff           @@
##             trunk    #261   +/-   ##
=======================================
  Coverage     7.96%   7.96%           
  Complexity    4285    4285           
=======================================
  Files          265     265           
  Lines        33076   33076           
=======================================
  Hits          2635    2635           
  Misses       30441   30441           

Flag	Coverage Δ
e2e	21.79% <ø> (ø)
phpunit	4.31% <ø> (ø)

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[github-actions]

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