chore: Release server.json schema version 2025-12-11 (#841)

Prepared by Claude Code, iterated and reviewed by me.

## Summary

This PR releases the server.json schema version `2025-12-11`,
incorporating URL template variables for remote servers from #570.

### What's New

- **URL Template Variables for Remote Servers** - Remote servers can now
define configurable variables in URLs using `{curly_braces}` notation,
enabling multi-tenant deployments.

### Schema Version
- `2025-10-17` → `2025-12-11`

## Release Sequence

1. ~~**First: Merge static repo PR** -
https://github.com/modelcontextprotocol/static/pull/11 (publishes schema
to versioned URL)~~
2. **Then: Merge this PR** - Updates registry code to reference the new
schema version

## Changes in This PR

- [x] Update CHANGELOG.md with release notes
- [x] Update `CurrentSchemaVersion` in `pkg/model/constants.go`
- [x] Update schema example URL in `docs/reference/api/openapi.yaml`
- [x] Update schema URL in `pkg/api/v0/types.go`
- [x] Update all documentation examples to use new schema version
- [x] Regenerate `server.schema.json`

## Test Plan
- [x] `make check` passes (lint, schema validation, example validation)
- [x] Unit tests pass for validators and publisher commands

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

docs/reference/server-json/CHANGELOG.md

@@ -16,6 +16,34 @@ When ready for release, changes in this section will be moved to a dated version
 
 ---
 
+## 2025-12-11
+
+### Changed
+
+#### URL Template Variables for Remote Servers ([#570](https://github.com/modelcontextprotocol/registry/pull/570))
+
+Remote servers can now define URL template variables using `{curly_braces}` notation. This enables multi-tenant deployments where a single server definition can support multiple endpoints with configurable values.
+
+**Example:**
+```json
+{
+  "remotes": [{
+    "type": "streamable-http",
+    "url": "https://{tenant_id}.api.example.com/mcp",
+    "variables": {
+      "tenant_id": {
+        "description": "Your tenant identifier",
+        "isRequired": true
+      }
+    }
+  }]
+}
+```
+
+**Migration:** No changes required. Existing servers continue to work unchanged.
+
+---
+
 ## 2025-10-17
 
 ### Changed