[MCP marketplace] Define curated registry contract and seed catalog

[GoDiao]

Parent: #12

Goal

Define a small, validated DreamCoder-curated MCP catalog that later UI and install work can consume safely.

Scope

• Add mcp-registry.json at the repository root.
• Define and document a versioned registry schema.
• Keep v1 explicitly limited to stdio servers.
• Seed 6–10 practical MCP servers.
• Add automated validation for the catalog and representative invalid entries.
• Document how contributors can add or update entries in docs/CONTRIBUTING_en.md and docs/CONTRIBUTING_zh.md.

Contract requirements

• Include stable identifier, display name, description, command, args, environment requirements, homepage, and tags.
• Environment declarations describe required variables and prompts; registry entries must never contain secret values.
• Package/runtime requirements and version-pinning policy must be explicit.
• Keep this curated install catalog distinct from the existing official MCP URL registry used for trust checks.

Done when

• Every seeded entry passes schema validation.
• Invalid or incomplete entries fail validation with actionable errors.
• Contributor documentation explains validation and review expectations.

Out of scope

• Browse UI.
• Writing to MCP configuration.
• HTTP/SSE catalog entries.