Add curated Postman skills

[nnandan-postman]

Summary

• add six curated Postman skills covering setup, build, docs, local workflows, readiness, and validation
• include shared Postman icons and agents/openai.yaml metadata for a consistent Codex picker experience
• add supporting references and a sample readiness report for the new workflows

Test plan

• Load the new skills locally through .agents/skills
• Verify the Postman icons render in the skill picker
• Exercise each skill end-to-end with a configured Postman MCP/CLI setup

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