feat: add advanced settings gate to agent creation wizard (#593)

* feat: add advanced settings gate to agent creation wizard

Insert a "Configure advanced settings?" Yes/No gate before network
configuration in both the Create and BYO agent paths. Defaults to
"No, use defaults" (Public network, no VPC), letting most users skip
network config with a single Enter press. Selecting "Yes, customize"
expands into the existing network mode, subnet, and security group
steps.

* style: fix prettier formatting

* docs: add TUI screenshots for advanced config gate

* test: add unit tests for advanced config gate wizard logic

15 tests covering step list composition, setAdvanced routing,
state cleanup on toggle, routing callbacks targeting advanced,
and reset behavior.

Author: tejaskash