feat(FN-1050): add per-agent custom instructions support

- Add instructionsPath and instructionsText fields to Agent type and AgentStore
- Create agent-instructions resolver module in engine with priority-based resolution
- Wire custom instructions into executor, triage, reviewer, and merger agents
- Add PATCH /agents/:id/instructions API endpoint with file and text support
- Add instructions editor UI to dashboard agent detail config tab
- Add comprehensive tests for instructions resolver and AgentStore integration
- Add changeset for published package bump

packages/dashboard/app/api.ts

@@ -1790,6 +1790,18 @@ export function updateAgent(agentId: string, updates: AgentUpdateInput, projectI
   });
 }
 
+/** Update agent custom instructions */
+export function updateAgentInstructions(
+  agentId: string,
+  instructions: { instructionsPath?: string; instructionsText?: string },
+  projectId?: string,
+): Promise<Agent> {
+  return api<Agent>(withProjectId(`/agents/${encodeURIComponent(agentId)}/instructions`, projectId), {
+    method: "PATCH",
+    body: JSON.stringify(instructions),
+  });
+}
+
 /** Update an agent's state */
 export function updateAgentState(agentId: string, state: AgentState, projectId?: string): Promise<Agent> {
   return api<Agent>(withProjectId(`/agents/${encodeURIComponent(agentId)}/state`, projectId), {