Add community guidance and review support for Business Central agents (#137)

* feat(community/agents): add AL agent quality guidance

- add 20 agent knowledge rules with good and bad AL samples
- clarify setup dialog shape, temporary persistence, permissions, profiles, instructions, capability registration, and interface wiring
- add the community-owned AL agents review skill
- make review fixture discovery layer-aware with custom, community, and Microsoft precedence
- document layer-aware evaluation behavior

* fix(community/agents): align setup and permission samples

- mark agent setup pages as non-extensible where required
- narrow the agent profile by hiding an unrelated sales-order field
- define a dedicated read-only permission set for the sales review agent
- assign AL-defined permission sets with system scope and the owning app ID
- clarify the permission scope guidance for default access controls

* Address agent review feedback

community/knowledge/agents/set-instructions-as-secrettext.bad.al

@@ -0,0 +1,19 @@
+codeunit 50100 "Sales Review Agent Create"
+{
+    procedure CreateWithInstructions()
+    var
+        Agent: Codeunit Agent;
+        TempAgentAccessControl: Record "Agent Access Control" temporary;
+        AgentUserSecurityId: Guid;
+        InstructionsLbl: Label 'You are a sales validation agent. Check credit.', Locked = true;
+    begin
+        AgentUserSecurityId := Agent.Create(
+            Enum::"Agent Metadata Provider"::"Sales Review Agent",
+            'SALESREVIEW',
+            'Sales Review Agent',
+            TempAgentAccessControl);
+        // Label/text is not SecretText and is type-wide, not per instance.
+        Agent.SetInstructions(AgentUserSecurityId, InstructionsLbl);
+        Agent.Activate(AgentUserSecurityId);
+    end;
+}