refactor: rename atlas agent to Atlas for naming consistency

- Rename agent name from 'atlas' to 'Atlas' (PascalCase like Sisyphus, Metis, Momus)
- Add migration for lowercase 'atlas' -> 'Atlas' for backward compatibility
- Keep hook name as 'atlas' (lowercase) to match other hook naming conventions
- Update all references in types, schema, hooks, commands, and tests

src/shared/migration.ts

@@ -18,7 +18,8 @@ export const AGENT_NAME_MAP: Record<string, string> = {
   librarian: "librarian",
   explore: "explore",
   "multimodal-looker": "multimodal-looker",
-  "orchestrator-sisyphus": "atlas",
+  "orchestrator-sisyphus": "Atlas",
+  atlas: "Atlas",
 }
 
 export const BUILTIN_AGENT_NAMES = new Set([
@@ -30,7 +31,7 @@ export const BUILTIN_AGENT_NAMES = new Set([
   "Metis (Plan Consultant)",
   "Momus (Plan Reviewer)",
   "Prometheus (Planner)",
-  "atlas",
+  "Atlas",
   "build",
 ])