feat(mcp): streamable HTTP transport with session, init, and dispatch

Implements the Streamable HTTP transport: POST routing handles initialize
(issues session ID), tools/list, tools/call, and unknown methods; GET SSE
emits a keepalive comment then blocks on context cancellation. A minimal
registry stub is introduced so the server compiles and tools/list returns
an empty array until Phase 6+ registers real tools.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/gitea-mcp/main.go

@@ -6,6 +6,8 @@ import (
 	"os"
 
 	"gitea.d-ma.be/mathias/gitea-mcp/internal/config"
+	"gitea.d-ma.be/mathias/gitea-mcp/internal/mcp"
+	"gitea.d-ma.be/mathias/gitea-mcp/internal/registry"
 )
 
 func main() {
@@ -17,14 +19,24 @@ func main() {
 		os.Exit(1)
 	}
 
+	reg := registry.New()
+	// Tool registration happens in Phase 6+; for now, registry is empty.
+
+	mcpSrv := mcp.NewServer(mcp.ServerOptions{
+		Registry:        reg,
+		OriginAllowlist: cfg.OriginAllowlist,
+		Sessions:        mcp.NewSessionStore(),
+	})
+
 	mux := http.NewServeMux()
+	mux.Handle("/mcp", mcpSrv)
 	mux.HandleFunc("/healthz", func(w http.ResponseWriter, _ *http.Request) {
 		w.WriteHeader(http.StatusOK)
 		_, _ = w.Write([]byte("ok"))
 	})
 
 	addr := ":" + cfg.Port
-	logger.Info("gitea-mcp starting", "addr", addr)
+	logger.Info("gitea-mcp starting", "addr", addr, "version", "0.1.0")
 	if err := http.ListenAndServe(addr, mux); err != nil {
 		logger.Error("server stopped", "err", err)
 		os.Exit(1)