docs: update supported openai model matrix

[leefowlercu]

What changed

This updates the public supported-model documentation to match the models we can currently evaluate and maintain:

• OpenAI GPT-5.6 Sol is now the only supported model.
• The normal Latest and N-1 operating model remains in place, but the N-1 row is temporarily omitted because GPT-5.5 is unavailable through AWS Bedrock.

The documentation also clarifies that OpenAI is the Model Provider while AWS Bedrock is the Inference Provider used for private Evaluations.

Affected surfaces

• Skills: no Skill source changes; the supported baseline applies to all 20 Skills
• Plugin Marketplaces: N/A
• Documentation: README.md, SUPPORTED_MODELS.md, and CHANGELOG.md
• Repository Metadata: N/A

Validation

• ./scripts/validate-structure.sh
• python3 scripts/check-links.py --local-only
• git diff --check

Ownership and review

• team-agent-skills-ecosystem

[leefowlercu]

Superseded by #100. The replacement starts from current main and carries the corrected Bedrock-evaluable model matrix, so this obsolete OpenAI-only history will not be merged.