Werkzeugkasten: Zugangsdaten je Einheit (#692)

A unit holds credentials for its members, by tool and setting - needed once,
not once per agent, and other units never get them.

- UnitConfig.credentials, protected by ConfigSecrets like every secret.
- OrgChart.ToolSettingsFor: an agent's own settings, completed by the nearest
  unit up that holds the value; own non-empty values win; no tool is granted.
  Used for runs, tool jobs and preflight probes.
- OrgAdministration.SetUnitCredential, PUT /organisation/units/{id}/credentials;
  the view lists held settings, never values.
- Templates never carry unit credentials; they are named instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/ClawdDotNet.Core.Tests/Security/SecretProtectorTests.cs

@@ -281,6 +281,24 @@ public sealed class ConfigSecretsTests : IDisposable
         agent.Tools["Mail"]["password"]!.ToString().ShouldNotBeNull().ShouldNotContain("mail-passwort");
     }
 
+    [Fact]
+    public void Credentials_a_unit_holds_are_protected_whatever_their_name()
+    {
+        // #692: everything a unit holds for its members is a secret.
+        var instance = new InstanceConfig();
+        instance.Units.Add(new UnitConfig
+        {
+            Id = "purchasing", Name = "Purchasing",
+            Credentials = new() { ["DirectAPI"] = new() { ["marketplaceKey"] = "mk-123" } }
+        });
+
+        ConfigSecrets.Protect(instance);
+        instance.Units[0].Credentials!["DirectAPI"]["marketplaceKey"].ShouldNotContain("mk-123");
+
+        ConfigSecrets.Unprotect(instance);
+        instance.Units[0].Credentials!["DirectAPI"]["marketplaceKey"].ShouldBe("mk-123");
+    }
+
     [Fact]
     public void Nicht_geheime_Felder_bleiben_lesbar()
     {