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/hooks/start-work/index.test.ts

@@ -379,7 +379,7 @@ describe("start-work hook", () => {
   })
 
   describe("session agent management", () => {
-    test("should update session agent to atlas when start-work command is triggered", async () => {
+    test("should update session agent to Atlas when start-work command is triggered", async () => {
       // #given
       const updateSpy = spyOn(sessionState, "updateSessionAgent")
       
@@ -395,7 +395,7 @@ describe("start-work hook", () => {
       )
 
       // #then
-      expect(updateSpy).toHaveBeenCalledWith("ses-prometheus-to-sisyphus", "atlas")
+      expect(updateSpy).toHaveBeenCalledWith("ses-prometheus-to-sisyphus", "Atlas")
       updateSpy.mockRestore()
     })
   })

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