refactor(agents): add prometheus agent and normalize agent key lookups

- Add 'prometheus' to BuiltinAgentNameSchema enum
- Update delegate_task parameter names in documentation (agent → subagent_type, background → run_in_background)
- Make agent name comparison case-insensitive in Atlas hook
- Implement case-insensitive agent config lookup in shared utilities
- Relax type signature for disabled agents parameter

🤖 Generated with assistance of OhMyOpenCode

src/shared/agent-variant.ts

@@ -1,4 +1,5 @@
 import type { OhMyOpenCodeConfig } from "../config"
+import { findCaseInsensitive } from "./case-insensitive"
 
 export function resolveAgentVariant(
   config: OhMyOpenCodeConfig,
@@ -11,7 +12,7 @@ export function resolveAgentVariant(
   const agentOverrides = config.agents as
     | Record<string, { variant?: string; category?: string }>
     | undefined
-  const agentOverride = agentOverrides?.[agentName]
+  const agentOverride = agentOverrides ? findCaseInsensitive(agentOverrides, agentName) : undefined
   if (!agentOverride) {
     return undefined
   }