Abilities integration: add Field abilities

[priethor]

What

Closes #171.

Adds Field abilities to the Abilities API.

Why

Fields are the core building blocks of SCF. Exposing field operations through the Abilities API allows AI agents to create, modify, and manage fields within field groups.

How

• Implementing the SCF_Field_Abilities class with full CRUD operations following the same pattern as the regular SCF internal post types.
• Doing some refactoring to the existing JSON schemas:
  • Extract the field schema to a separate field.schema.json file for reusability between field groups that contain fields and standalone fields; the field group schema now references the field schema for its fields array.
  • Refactor the internal fields schema (the ones that are stripped during export), as Fields only have ID and _valid but not local or not_registered, since they are not internal post types.

Testing Instructions

1. Connect to SCF through MCP
2. Ask your AI agent of choice to create/modify a field, list them by type text, etc.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 99.51456% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 14.35%. Comparing base (de6ee27) to head (4ebb80f).
⚠️ Report is 1 commits behind head on trunk.

Files with missing lines	Patch %	Lines
includes/abilities/class-scf-field-abilities.php	99.50%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #300      +/-   ##
============================================
+ Coverage     13.40%   14.35%   +0.95%     
- Complexity     4321     4386      +65     
============================================
  Files           285      286       +1     
  Lines         34863    35270     +407     
============================================
+ Hits           4674     5064     +390     
- Misses        30189    30206      +17     

Flag	Coverage Δ
e2e	30.69% <ø> (-0.03%)	⬇️
javascript	2.60% <ø> (ø)
phpunit	10.47% <99.51%> (+1.32%)	⬆️

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