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>

go.mod

@@ -6,7 +6,7 @@ toolchain go1.26.5
 
 require (
 	gitea.dev/sdk v1.2.0
-	github.com/mark3labs/mcp-go v0.57.0
+	github.com/modelcontextprotocol/go-sdk v1.7.0
 	go.uber.org/zap v1.28.0
 	go.uber.org/zap/exp v0.3.0
 	gopkg.in/natefinch/lumberjack.v2 v2.2.1
@@ -16,13 +16,15 @@ require (
 	github.com/42wim/httpsig v1.2.4 // indirect
 	github.com/davidmz/go-pageant v1.0.2 // indirect
 	github.com/google/jsonschema-go v0.4.3 // indirect
-	github.com/google/uuid v1.6.0 // indirect
 	github.com/hashicorp/go-version v1.9.0 // indirect
-	github.com/santhosh-tekuri/jsonschema/v6 v6.0.2 // indirect
-	github.com/spf13/cast v1.10.0 // indirect
+	github.com/segmentio/asm v1.1.3 // indirect
+	github.com/segmentio/encoding v0.5.4 // indirect
 	github.com/yosida95/uritemplate/v3 v3.0.2 // indirect
 	go.uber.org/multierr v1.11.0 // indirect
 	golang.org/x/crypto v0.54.0 // indirect
+	golang.org/x/oauth2 v0.35.0 // indirect
+	golang.org/x/sync v0.22.0 // indirect
 	golang.org/x/sys v0.47.0 // indirect
-	golang.org/x/text v0.40.0 // indirect
+	golang.org/x/time v0.15.0 // indirect
+	golang.org/x/tools v0.47.0 // indirect
 )