Add example servers for SEP-2053 variant patterns (#4)

Add four new example servers demonstrating different SEP-2053 variant
patterns:

- model-optimized (same tools, different descriptions per LLM family)
- trading (API versioning with deprecation lifecycle)
- research (context budget management)
- Refactor the existing variants-http example into a GitHub-style server
with custom ranking.

Signed-off-by: Kurt Degiorgio <kdegiorgio@bloomberg.net>

go/sdk/examples/server/github/README.md

@@ -0,0 +1,30 @@
+# GitHub Developer Platform
+
+A variant-aware MCP server that exposes different tool sets for different agent types, mirroring the GitHub MCP Server pattern described in SEP-2053's Prior Art section.
+
+**Pattern demonstrated:** Different tool sets per variant with custom ranking.
+
+## Variants
+
+| Variant | Tools | Description |
+|---|---|---|
+| `code-review` | `list_pull_requests`, `get_diff`, `add_review_comment` | PR operations and code review |
+| `project-management` | `list_issues`, `create_issue`, `add_label` | Issue tracking and labels |
+| `security-readonly` | `list_security_alerts`, `get_advisory` | Security scanning (read-only) |
+| `ci-automation` | `list_workflow_runs`, `trigger_workflow` | CI/CD workflow management |
+
+## Custom Ranking
+
+Clients send a `"domain"` hint during initialization. The ranking function boosts variants whose `domain` hint matches the client's requested domain, falling back to priority order.
+
+## Run
+
+```bash
+go run ./examples/server/github
+```
+
+Connect any MCP client to `http://localhost:8080`.
+
+## Demo
+
+See [mcp-inspector-variants-demo.mp4](mcp-inspector-variants-demo.mp4) for a walkthrough using MCP Inspector.