refactor!: replace mcp-go with the official MCP Go SDK

- Swap the mark3labs MCP dependency for modelcontextprotocol/go-sdk v1.7.0
- Add a declarative tool definition and JSON Schema builder to the tool package
- Adapt registered tools to the official low-level handler, recovering panics and mapping failures to JSON-RPC errors
- Narrow tool handlers to take a plain argument map instead of an SDK request type
- Rewire stdio and HTTP transports onto the official server, moving Authorization parsing into receiving middleware
- Add a golden contract test that locks the exposed tool schemas, plus SDK integration and helper tests
- Add a test target and run it in the pull request workflow

BREAKING CHANGE: The exported helpers change signature. Tool.RegisterRead and
Tool.RegisterWrite now take tool.ServerTool instead of server.ServerTool, and the
annotation constructors return *mcp.ToolAnnotations from the official SDK. Callers
must build tool definitions with tool.NewDefinition and handlers with the
func(context.Context, map[string]any) signature.

The 30-second SSE heartbeat is removed because the official SDK has no equivalent.
ServerOptions.KeepAlive is deliberately not used as a substitute, since it sends MCP
ping requests and ping is removed in protocol 2026. HTTP stays stateless=false, so
new clients negotiate at most 2025-11-25.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

go.mod

@@ -4,7 +4,7 @@ go 1.26.0
 
 require (
 	gitea.dev/sdk v1.2.0
-	github.com/mark3labs/mcp-go v0.56.0
+	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
@@ -14,13 +14,15 @@ 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
 )