fix(mcp): mount /mcp/rpc with explicit method patterns

Go 1.22 ServeMux treats prefix patterns as conflicting with the root
catch-all when one matches more methods than the other. Switch from
prefix mounting (`/mcp/`) to two explicit `POST/GET /mcp/rpc` handlers
so the projax server boots cleanly. /healthz crash loop on first deploy
of phase 3a was caused by this.

web/server.go

@@ -136,7 +136,12 @@ func (s *Server) Routes() http.Handler {
 	})
 
 	if s.MCP != nil {
-		mux.Handle("/mcp/", http.StripPrefix("/mcp", s.MCP))
+		// Mount MCP routes with explicit method+path patterns. A prefix pattern
+		// like `/mcp/` would conflict with `GET /` under Go 1.22's strict
+		// ServeMux (the prefix matches more methods than the subtree root).
+		mcpHandler := http.StripPrefix("/mcp", s.MCP)
+		mux.Handle("POST /mcp/rpc", mcpHandler)
+		mux.Handle("GET /mcp/rpc", mcpHandler)
 	}
 
 	static, _ := fs.Sub(staticFS, "static")