feat: enforce agent icon enum and expose via LLM endpoint

Move icon name list to shared constants with strict enum validation.
Add /llms/agent-icons.txt endpoint, pass icon through hire flow,
and update skills to reference icon discovery step.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/routes/agents.ts

@@ -512,6 +512,7 @@ export function agentRoutes(db: Db) {
           name: normalizedHireInput.name,
           role: normalizedHireInput.role,
           title: normalizedHireInput.title ?? null,
+          icon: normalizedHireInput.icon ?? null,
           reportsTo: normalizedHireInput.reportsTo ?? null,
           capabilities: normalizedHireInput.capabilities ?? null,
           adapterType: requestedAdapterType,