Rename mcpserver package to extension, remove *mcp.Server field (#11)

## Summary

- Rename `interceptors/mcpserver/` → `interceptors/extension/`,
replacing the `Server` wrapper with a lightweight `Extension` that can
install itself on any `*mcp.Server`
- Remove the stored `*mcp.Server` field — `Extension` is now a pure
interceptor container with no server reference
- Add `Install(server)` method to register JSON-RPC methods and init
middleware on a given server
- Update `LocalChain(ctx)` → `LocalChain(ctx, server)` to accept the
server as a parameter
- Remove `MCPServer()`, `Run()`, and `NewStreamableHTTPHandler()`
wrapper methods — users interact with `mcp.Server` directly
- Update all examples, tests, and documentation to use the new API

## Why

`mcpserver.Server` pretended to *be* the server — it had `Run()`,
`NewStreamableHTTPHandler()`, etc. — but it could never cover the full
`*mcp.Server` API, so users constantly needed `MCPServer()` to unwrap
it.

`Extension` doesn't pretend to be the server. It's a pure interceptor
container that can install itself on any server (or multiple servers)
on the fly. No stored server reference, no wrapping, no delegation
surface to maintain.

### Before

```go
mcpServer := mcp.NewServer(...)
srv := mcpserver.NewServer(mcpServer)
srv.AddInterceptor(v)
chain, err := srv.LocalChain(ctx)
```

### After

```go
mcpServer := mcp.NewServer(...)
ext := extension.New()
ext.AddInterceptor(v)
ext.Install(mcpServer)
chain, err := ext.LocalChain(ctx, mcpServer)
```

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

go/sdk/examples/mutator/main.go

@@ -15,8 +15,8 @@ import (
 	"github.com/modelcontextprotocol/go-sdk/mcp"
 
 	"github.com/modelcontextprotocol/ext-interceptors/go/sdk/interceptors"
+	"github.com/modelcontextprotocol/ext-interceptors/go/sdk/interceptors/extension"
 	"github.com/modelcontextprotocol/ext-interceptors/go/sdk/interceptors/integrations/gomiddleware"
-	"github.com/modelcontextprotocol/ext-interceptors/go/sdk/interceptors/mcpserver"
 )
 
 func main() {
@@ -73,13 +73,14 @@ func main() {
 		},
 	}
 
-	// Create the interceptor server (registers interceptors as discoverable
+	// Create the interceptor extension (registers interceptors as discoverable
 	// resources via interceptors/list and interceptor/invoke).
-	srv := mcpserver.NewServer(mcpServer)
-	srv.AddInterceptor(m)
+	ext := extension.New()
+	ext.AddInterceptor(m)
+	ext.Install(mcpServer)
 
 	// Create a chain connected via in-memory transport.
-	chain, err := srv.LocalChain(context.Background())
+	chain, err := ext.LocalChain(context.Background(), mcpServer)
 	if err != nil {
 		log.Fatal(err)
 	}
@@ -101,7 +102,10 @@ func main() {
 		),
 	)
 
-	handler := mcpserver.NewStreamableHTTPHandler(srv, nil)
+	handler := mcp.NewStreamableHTTPHandler(
+		func(r *http.Request) *mcp.Server { return mcpServer },
+		nil,
+	)
 	log.Println("listening on :8080")
 	if err := http.ListenAndServe(":8080", handler); err != nil {
 		log.Fatal(err)