feat: add client capability schema

PR-DESCRIPTION.md

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+## Summary
+
+Adds a JSON Schema for MCP client capabilities, providing a formal structure for documenting what features each client supports.
+
+Closes #718
+
+## Acknowledgments
+
+This schema builds on the work done by @jancurn in [mcp-client-capabilities](https://github.com/apify/mcp-client-capabilities), which established the core capability structure and has catalogued 40+ clients. This PR extends that foundation with additional fields for transports, platforms, authentication, and metadata.
+
+## Changes
+
+- **`docs/schemas/client-registry.json`** - JSON Schema 2020-12 definition
+
+## What's Included
+
+This PR focuses solely on the **schema definition**. Intentionally not included:
+
+- **Client data** - The mcp-client-capabilities repo already has the data. Once the schema is agreed upon, that data can be migrated.
+- **Validation scripts** - Can be added in a follow-up PR once the schema is finalized.
+
+## Schema Overview
+
+The schema tracks BOTH what clients declare to servers (sampling, elicitation, roots) AND which server capabilities clients can consume (tools, resources, prompts). This is intentionally broader than the MCP spec's `ClientCapabilities` interface.
+
+**Minimal entry:**
+```json
+{
+  "my-client": {
+    "title": "My Client",
+    "url": "https://example.com",
+    "protocolVersion": "2025-11-25"
+  }
+}
+```
+
+**Full entry:**
+```json
+{
+  "claude-code": {
+    "title": "My Advanced Client",
+    "url": "https://example.com",
+    "protocolVersion": "2025-11-25",
+    "category": "cli",
+    "platforms": ["windows", "macos", "linux"],
+    "transports": { "stdio": {} },
+    "tools": { "listChanged": true },
+    "resources": { "listChanged": true, "subscribe": true },
+    "prompts": { "listChanged": true },
+    "sampling": { "context": {}, "tools": {} },
+    "elicitation": { "form": {}, "url": {} },
+    "roots": { "listChanged": true },
+    "tasks": {
+      "list": {},
+      "cancel": {},
+      "requests": { "sampling": { "createMessage": {} } }
+    },
+    "metadata": {
+      "vendor": "Some Vendor",
+      "lastVerified": "2026-01-15"
+    }
+  }
+}
+```
+
+## Features
+
+| Feature | Description |
+|---------|-------------|
+| **Capabilities** | tools, resources, prompts, sampling, elicitation, roots, completions, logging, instructions, tasks |
+| **Status values** | `{}` (supported), or `{ "status": "full\|partial\|unsupported\|untested", "notes": "..." }` |
+| **Categories** | ide, chat, cli, automation, browser-extension, library, other |
+| **Platforms** | windows, macos, linux, web, ios, android |
+| **Transports** | stdio, sse, streamableHttp |
+| **Auth** | oauth (with PKCE, grantTypes), header, none |
+| **Metadata** | vendor, openSource, sourceUrl, license, lastVerified, conformanceScore |
+
+## Related
+
+- [modelcontextprotocol#1814](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1814) - Client compatibility matrix
+- [mcp-client-capabilities](https://github.com/apify/mcp-client-capabilities) - Community client data
+
+---
+
+🦉 Generated with [Claude Code](https://claude.ai/code) and reviewed by me