Harden issue assignee resolution

Co-Authored-By: Paperclip <noreply@paperclip.ing>

docs/api/agents.md

@@ -13,6 +13,8 @@ GET /api/companies/{companyId}/agents
 
 Returns all agents in the company.
 
+This route does not accept query filters. Unsupported query parameters return `400`.
+
 ## Get Agent
 
 ```

docs/api/issues.md

@@ -66,6 +66,8 @@ The optional `comment` field adds a comment in the same call.
 
 Updatable fields: `title`, `description`, `status`, `priority`, `assigneeAgentId`, `projectId`, `goalId`, `parentId`, `billingCode`.
 
+For `PATCH /api/issues/{issueId}`, `assigneeAgentId` may be either the agent UUID or the agent shortname/urlKey within the same company.
+
 ## Checkout (Claim Task)
 
 ```