v6.0.0 — Codex-native skill packaging

v6.0.0 — Codex-native skill packaging

What changed

• Added Codex skill UI metadata via agents/openai.yaml.
• Added Codex plugin metadata via .codex-plugin/plugin.json.
• Added skills/mathmodel-skill/SKILL.md as a thin plugin shim so the plugin follows the official ./skills/ layout while keeping the root SKILL.md as the source of truth.
• Updated SKILL.md, AGENTS.md, references/harness_compat.md, and README.md for Codex-native skill discovery, project-level AGENTS.md, plugin distribution, and .agents/skills/ installation.
• Updated README version badges and development log to V6.

OpenAI docs alignment

• AGENTS.md is now documented as project/workspace instructions, not the only Codex entry point.
• SKILL.md frontmatter remains the primary trigger surface for Codex Skills.
• Plugin packaging now uses the official ./skills/ layout described in the Codex plugin docs.

Compatibility

• Runtime workflow, scoring scripts, templates, competition data, and decision_log.json schema are unchanged.
• Existing Claude Code and Codex handoff behavior remains compatible.

Validation

• Root skill validation passed.
• Plugin shim skill validation passed.
• score_artifact.py smoke passed for CUMCM stage 1, MCM stage 3, and Diangong stage 5 using temporary initialized state.