fix: strengthen group chat collaboration guidance

Audit confirmed that single, sequential, concurrent, and handoff are mapped to distinct workflow builders as intended. Group-chat was also using the correct round-robin manager, but the agents only had one-shot role prompts, so the reviewer could behave like a fresh assistant instead of refining the draft in progress. Add explicit runtime guidance for group-chat participants plus clearer built-in Writer/Reviewer instructions and regression coverage.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/shared/domain/pattern.ts

@@ -204,7 +204,8 @@ export function createBuiltinPatterns(timestamp: string): PatternDefinition[] {
           id: 'agent-group-writer',
           name: 'Writer',
           description: 'Produces candidate answers.',
-          instructions: 'You draft a concise, useful answer for the task.',
+          instructions:
+            'You draft a concise, useful answer for the task. On later turns, refine your earlier draft based on peer feedback rather than restarting.',
           model: defaultModels.gpt54,
           reasoningEffort: 'medium',
         },
@@ -212,7 +213,8 @@ export function createBuiltinPatterns(timestamp: string): PatternDefinition[] {
           id: 'agent-group-reviewer',
           name: 'Reviewer',
           description: 'Critiques and refines the answer.',
-          instructions: 'You review the latest answer and improve it when needed.',
+          instructions:
+            'You review the latest draft and offer specific improvements. Focus on critique and refinement instead of restarting the conversation.',
           model: defaultModels.claude,
           reasoningEffort: 'medium',
         },