Add Full-Vibe/ucp-ecosystem and ucplist.ai to Developer Tools

[JakeSc]

What this adds

Two related entries under Developer Tools:

1. UCP Ecosystem Registry — An open data registry of UCP-enabled merchants, tools, and platforms. Data is stored as JSON (listings.json, categories.json, schema.json) and community-editable via PR. Licensed CC BY 4.0.

2. ucplist.ai — The web interface built on top of the ucp-ecosystem registry. Searchable directory of UCP resources with a submission form at ucplist.ai/submit.

Why these belong here

• Both are community resources focused on discoverability of UCP implementations
• ucp-ecosystem is a programmatic data source (JSON) that developers can query or contribute to directly
• ucplist.ai provides a human-browseable and agent-queryable interface to that same data
• Neither is a validator, SDK, or merchant integration — closest fit is Developer Tools alongside the existing directory-style entries

Checklist

• Entries are alphabetically consistent with section (not strictly required by existing pattern)
• Links are live and publicly accessible
• Descriptions are factual and follow existing style (no marketing language)
• One-line description format matches existing entries