refactor: replace mcp-go with the official MCP Go SDK (#223)

## Summary

Replace `github.com/mark3labs/mcp-go` with the official
`github.com/modelcontextprotocol/go-sdk v1.7.0`.

All 54 existing tools, scopes, CLI, `stdio` and HTTP (`/mcp`) modes, and
per-request authentication remain supported. This PR is limited to the SDK
equivalence migration; the stateless 2026 HTTP transport is deferred to a
follow-up PR.

## Verification

- Tool definitions were compared with `main` and matched for all 54 tools.
- `make lint`, `make fmt`, `go test -count=1 -race ./...`, `make build`, and
  `make tidy` pass.

---------

Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/223
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: Bo-Yi Wu <appleboy.tw@gmail.com>

pkg/to/to.go

@@ -7,7 +7,7 @@ import (
 	"gitea.com/gitea/gitea-mcp/pkg/flag"
 	"gitea.com/gitea/gitea-mcp/pkg/log"
 
-	"github.com/mark3labs/mcp-go/mcp"
+	"github.com/modelcontextprotocol/go-sdk/mcp"
 )
 
 func TextResult(v any) (*mcp.CallToolResult, error) {
@@ -18,7 +18,9 @@ func TextResult(v any) (*mcp.CallToolResult, error) {
 	if flag.Debug {
 		log.Debugf("Text Result: %s", string(resultBytes))
 	}
-	return mcp.NewToolResultText(string(resultBytes)), nil
+	return &mcp.CallToolResult{
+		Content: []mcp.Content{&mcp.TextContent{Text: string(resultBytes)}},
+	}, nil
 }
 
 func ErrorResult(err error) (*mcp.CallToolResult, error) {