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/atlas/index.ts

@@ -498,7 +498,7 @@ export function createAtlasHook(
        await ctx.client.session.prompt({
          path: { id: sessionID },
          body: {
-            agent: "Atlas",
+            agent: "atlas",
            ...(model !== undefined ? { model } : {}),
            parts: [{ type: "text", text: prompt }],
          },