feat(FN-1559): enforce feature status invariants at API boundary

- Guard PATCH /api/missions/features/:featureId to reject status
  transitions to execution states (triaged, in-progress, done, blocked)
  when feature has no taskId
- 'defined' status remains always allowed (initial state)
- Non-status field updates (title, description) are unaffected
- Add 6 new tests covering guard behavior and edge cases
- All 147 mission-e2e tests passing

packages/core/src/prompt-overrides.ts

@@ -36,7 +36,9 @@ export type PromptKey =
   | "reviewer-verdict"
   | "merger-conflicts"
   | "agent-generation-system"
-  | "workflow-step-refine";
+  | "workflow-step-refine"
+  | "planning-system"
+  | "subtask-breakdown-system";
 
 /**
  * Metadata describing a prompt key including its purpose and default content.
@@ -251,6 +253,86 @@ The prompt should:
 
 Output ONLY the prompt text (no markdown, no explanations).`,
   },
+  "planning-system": {
+    key: "planning-system",
+    name: "Planning System",
+    roles: ["triage"],
+    description: "System prompt for the AI planning assistant that guides users through task definition",
+    defaultContent: `You are a planning assistant for the fn task board system.
+
+Your job: help users transform vague, high-level ideas into well-defined, actionable tasks.
+
+## Conversation Flow
+1. User provides a high-level plan (e.g., "Build a user auth system")
+2. You ask clarifying questions to understand scope, requirements, and constraints
+3. You present UI-friendly selection options when appropriate
+4. Once you have enough information, generate a structured summary
+
+## Question Types to Use
+- "text": Open-ended follow-up questions for detailed input
+- "single_select": When user must choose one option (e.g., tech stack preference)
+- "multi_select": When multiple options can apply (e.g., features to include)
+- "confirm": Yes/No questions for quick decisions
+
+## Guidelines
+- Ask 3-7 questions depending on complexity
+- Start broad, then narrow down specifics
+- Suggest sensible defaults based on project context
+- Keep questions focused and actionable
+- When asking about file scope, reference actual project structure
+
+## Summary Generation
+When ready to complete, generate:
+- A concise but descriptive title (max 80 chars)
+- A detailed description with context gathered
+- Size estimate (S/M/L) based on scope
+- Any suggested dependencies on existing tasks
+- Key deliverables as a checklist
+
+## Response Format
+Always respond with valid JSON in one of these formats:
+
+For questions:
+{\n  "type": "question",\n  "data": {\n    "id": "unique-id",\n    "type": "text|single_select|multi_select|confirm",\n    "question": "The question text",\n    "description": "Helpful context",\n    "options": [{"id": "opt1", "label": "Option 1", "description": "Details"}]\n  }\n}
+
+For completion:
+{\n  "type": "complete",\n  "data": {\n    "title": "Task title",\n    "description": "Detailed description",\n    "suggestedSize": "S|M|L",\n    "suggestedDependencies": [],\n    "keyDeliverables": ["Item 1", "Item 2"]\n  }\n}`,
+  },
+  "subtask-breakdown-system": {
+    key: "subtask-breakdown-system",
+    name: "Subtask Breakdown System",
+    roles: ["executor"],
+    description: "System prompt for the AI subtask decomposition assistant",
+    defaultContent: `You are a task decomposition assistant for the fn task board system.
+
+Analyze the user's task description and break it down into 2-5 smaller, independently executable subtasks.
+
+For each subtask, provide:
+1. Title (short and descriptive)
+2. Description (1-2 sentences, implementation-focused)
+3. Size estimate (S: <2h, M: 2-4h, L: 4-8h)
+4. Dependencies (which other subtask IDs must be completed first)
+
+Guidelines:
+- Prefer parallelizable subtasks when possible
+- Only add dependencies when truly required
+- Order subtasks so prerequisites appear earlier
+- Keep the overall scope aligned with the original task
+- Use IDs like "subtask-1", "subtask-2", etc.
+
+Return ONLY valid JSON in this format:
+{
+  "subtasks": [
+    {
+      "id": "subtask-1",
+      "title": "...",
+      "description": "...",
+      "suggestedSize": "S",
+      "dependsOn": []
+    }
+  ]
+}`,
+  },
 };
 
 /**