fix(app): show picker for custom agents - #37138

Summary

• show the agent picker automatically when any selectable agent exists beyond Build and Plan
• keep agent cycle commands available in the same state
• preserve the explicit setting for users who want the picker with only default agents

Checks

• bun typecheck (packages/app)
• git diff --check

Test note

• targeted session-composer-state.test.ts currently fails before running tests because Solid Router invokes a client-only API under the direct server-side test command