fix(agents): use lowercase agent names in API calls

- atlas/index.ts: agent: 'Atlas' -> 'atlas'
- start-work/index.ts: updateSessionAgent(..., 'Atlas') -> 'atlas'
- builtin-commands/commands.ts: agent: 'Atlas' -> 'atlas'
- Updated tests to match lowercase convention

src/hooks/start-work/index.ts

@@ -71,7 +71,7 @@ export function createStartWorkHook(ctx: PluginInput) {
         sessionID: input.sessionID,
       })
 
-      updateSessionAgent(input.sessionID, "Atlas")
+      updateSessionAgent(input.sessionID, "atlas")
 
       const existingState = readBoulderState(ctx.directory)
       const sessionId = input.sessionID