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>

Makefile

@@ -6,6 +6,8 @@ LDFLAGS := -X "main.Version=$(VERSION)"
 GOLANGCI_LINT_PACKAGE ?= github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.12.2 # renovate: datasource=go
 GOVULNCHECK_PACKAGE ?= golang.org/x/vuln/cmd/govulncheck@v1.6.0 # renovate: datasource=go
 
+GOTEST_FLAGS ?= -race -timeout 20m
+
 .PHONY: help
 help: ## print this help message
 	@echo "Usage: make [target]"
@@ -38,6 +40,10 @@ clean: ## delete build artifacts
 build: ## build the application
 	$(GO) build -v -ldflags '-s -w $(LDFLAGS)' -o $(EXECUTABLE)
 
+.PHONY: test
+test: ## run Go tests
+	$(GO) test $(GOTEST_FLAGS) ./...
+
 .PHONY: air
 air: ## install air for hot reload
 	@hash air > /dev/null 2>&1; if [ $$? -ne 0 ]; then \