feat(epic): add Working Model as required section for epics

Epics now require a "Working Model" section in their description,
in addition to "Success Criteria". This provides clear guidance on
HOW the epic will be executed:

- Owner role: Coordinator vs Implementer
- Delegation target: Polecats, crew, external
- Review process: Approval gates

Closes gt-0lp

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/validation/template_test.go

@@ -97,19 +97,34 @@ Widget displays correctly`,
 
 		// Epic type tests
 		{
-			name:      "epic with success criteria",
+			name:      "epic with all sections",
 			issueType: types.TypeEpic,
 			description: `Big project
 
 ## Success Criteria
 - Project ships
-- Users happy`,
+- Users happy
+
+## Working Model
+- Owner role: Coordinator
+- Delegation target: Polecats
+- Review process: Refinery MQ`,
 			wantErr: false,
 		},
 		{
-			name:        "epic missing success criteria",
+			name:        "epic missing all sections",
 			issueType:   types.TypeEpic,
 			description: "Do everything",
+			wantErr:     true,
+			wantMissing: 2,
+		},
+		{
+			name:      "epic missing working model",
+			issueType: types.TypeEpic,
+			description: `Big project
+
+## Success Criteria
+- Project ships`,
 			wantErr:     true,
 			wantMissing: 1,
 		},