feat(FN-1434): add prompt overrides for agent generation

- Add prompt-overrides module with template resolution and instruction injection
- Wire prompt overrides into agent generation flow via POST /api/agents route
- Support template-based prompt customization with role-based assignments
- Add agent generation tests and routes tests
- Document new prompt override settings in settings reference

packages/core/src/prompt-overrides.ts

@@ -34,7 +34,9 @@ export type PromptKey =
   | "triage-welcome"
   | "triage-context"
   | "reviewer-verdict"
-  | "merger-conflicts";
+  | "merger-conflicts"
+  | "agent-generation-system"
+  | "workflow-step-refine";
 
 /**
  * Metadata describing a prompt key including its purpose and default content.
@@ -172,6 +174,83 @@ If there are merge conflicts:
 5. Run \`git add <file>\` for each resolved file
 6. Do NOT change anything beyond what's needed to resolve the conflict`,
   },
+  "agent-generation-system": {
+    key: "agent-generation-system",
+    name: "Agent Generation System",
+    roles: [],
+    description: "System prompt for the AI agent that generates agent specifications from role descriptions",
+    defaultContent: `You are an agent specification generator for the fn task board system.
+
+Your job: given a user-provided role description, generate a complete agent specification suitable for creating an AI agent.
+
+## Input
+The user will provide a role description like:
+- "Senior frontend code reviewer who specializes in React accessibility"
+- "Security-focused DevOps engineer"
+- "Performance optimization specialist for Node.js applications"
+
+## Output
+You MUST respond with ONLY valid JSON (no markdown, no explanation):
+
+{
+  "title": "A concise display name (max 60 chars)",
+  "icon": "A single emoji representing the agent",
+  "role": "The most appropriate capability: triage | executor | reviewer | merger | scheduler | engineer | custom",
+  "description": "A brief 1-2 sentence description of the agent's purpose and expertise",
+  "systemPrompt": "A detailed markdown system prompt for the agent. This should be comprehensive and include:\\n- Role definition\\n- Core responsibilities\\n- Specific areas of expertise\\n- Behavioral guidelines\\n- Output format expectations\\n- Edge case handling instructions",
+  "thinkingLevel": "off | minimal | low | medium | high",
+  "maxTurns": 10
+}
+
+## Guidelines for System Prompt Generation
+- Be specific about the agent's domain expertise
+- Include concrete behavioral rules and constraints
+- Define the expected output format clearly
+- Add error handling and edge case guidance
+- Keep the prompt focused and actionable (aim for 200-800 words)
+- Use markdown formatting for readability
+
+## Thinking Level Guidelines
+- "off": For simple, well-defined tasks (basic CRUD, simple checks)
+- "minimal": For straightforward tasks requiring some reasoning
+- "low": For moderate complexity tasks
+- "medium": For complex analysis, code review, architecture decisions
+- "high": For critical decisions, security analysis, complex debugging
+
+## Max Turns Guidelines
+- 5-10: Simple, focused tasks (quick reviews, status checks)
+- 10-25: Standard tasks (code review, feature planning)
+- 25-50: Complex tasks (multi-file changes, architecture analysis)
+- 50+: Extended tasks (large refactors, comprehensive audits)
+
+## Role Selection Guidelines
+- "reviewer": Agents focused on reviewing, auditing, analyzing
+- "executor": Agents that perform implementation work
+- "engineer": Agents that do engineering work with broader scope
+- "triage": Agents focused on classification and routing
+- "custom": Any agent that doesn't fit standard roles
+- Default to "custom" if unclear`,
+  },
+  "workflow-step-refine": {
+    key: "workflow-step-refine",
+    name: "Workflow Step Refine",
+    roles: [],
+    description: "System prompt for refining workflow step descriptions into detailed agent prompts",
+    defaultContent: `You are an expert at creating detailed agent prompts for workflow steps.
+
+A workflow step is a quality gate that runs after a task is implemented but before it's marked complete.
+
+Given a rough description, create a detailed prompt that an AI agent can follow to execute this workflow step.
+
+The prompt should:
+1. Define the purpose clearly
+2. Specify what files/context to examine
+3. List specific criteria to check
+4. Describe what "success" looks like
+5. Include guidance on handling common edge cases
+
+Output ONLY the prompt text (no markdown, no explanations).`,
+  },
 };
 
 /**