Add third-party self-referential relations

Introduce a self-referential many-to-many relation table so a
third party can have child third parties. Each relation is
directional (parent to child); both directions can coexist as
independent rows.

Add a first_level boolean on third_parties (default true) with
a filter on the list page that defaults to showing only
first-level third parties.

Frontend adds a "Third Parties" tab on the detail page where
users can link existing third parties or create new ones from
the common third party catalog (created as non-first-level).
The list page gets a First Level/All toggle filter.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

pkg/cmd/thirdpartymgmt/link/link.go

@@ -0,0 +1,108 @@
+// Copyright (c) 2025-2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package link
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"github.com/spf13/cobra"
+	"go.probo.inc/probo/pkg/cli/api"
+	"go.probo.inc/probo/pkg/cmd/cmdutil"
+)
+
+const linkMutation = `
+mutation($input: CreateThirdPartyThirdPartyMappingInput!) {
+  createThirdPartyThirdPartyMapping(input: $input) {
+    thirdPartyEdge {
+      node {
+        id
+        name
+      }
+    }
+  }
+}
+`
+
+type linkResponse struct {
+	CreateThirdPartyThirdPartyMapping struct {
+		ThirdPartyEdge struct {
+			Node struct {
+				ID   string `json:"id"`
+				Name string `json:"name"`
+			} `json:"node"`
+		} `json:"thirdPartyEdge"`
+	} `json:"createThirdPartyThirdPartyMapping"`
+}
+
+func NewCmdLink(f *cmdutil.Factory) *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "link <parent-id> <child-id>",
+		Short: "Link a child thirdParty to a parent thirdParty",
+		Example: `  # Link a child third_party to a parent
+  prb thirdParty link <parent-id> <child-id>`,
+		Args: cobra.ExactArgs(2),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			cfg, err := f.Config()
+			if err != nil {
+				return err
+			}
+
+			host, hc, err := cfg.DefaultHost()
+			if err != nil {
+				return err
+			}
+
+			client := api.NewClient(
+				host,
+				hc.Token,
+				"/api/console/v1/graphql",
+				cfg.HTTPTimeoutDuration(),
+				cmdutil.TokenRefreshOption(cfg, host, hc),
+			)
+
+			data, err := client.Do(
+				linkMutation,
+				map[string]any{
+					"input": map[string]any{
+						"parentThirdPartyId": args[0],
+						"childThirdPartyId":  args[1],
+					},
+				},
+			)
+			if err != nil {
+				return err
+			}
+
+			var resp linkResponse
+			if err := json.Unmarshal(data, &resp); err != nil {
+				return fmt.Errorf("cannot parse response: %w", err)
+			}
+
+			v := resp.CreateThirdPartyThirdPartyMapping.ThirdPartyEdge.Node
+			_, _ = fmt.Fprintf(
+				f.IOStreams.Out,
+				"Linked thirdParty %s (%s) as child of %s\n",
+				v.ID,
+				v.Name,
+				args[0],
+			)
+
+			return nil
+		},
+	}
+
+	return cmd
+}