[codex] add naxytra discovery agent entry

[naxytra]

What changed

• Added agents/naxytra-machine-discovery-agent/agent_card.json as a repo-native A2A contribution.
• Added a short folder README that points to the canonical naxytra discovery surfaces.

Why

This repo's CONTRIBUTING guide asks contributors to open a PR with their card and an agent folder under agents/<agent-id>. This PR follows that path and provides a public discovery entry for the live naxytra stack.

User impact

• Gives the community a concrete agent directory entry to reference.
• Points builders to the canonical machine entry, the A2A card, and the MCP remote URL.
• Keeps the contribution aligned with the repo's own federation guidance.

Validation

• Parsed agent_card.json locally with PowerShell ConvertFrom-Json.
• Confirmed the branch is pushed to the naxytra/a2a-gateway fork.