Custom steps should support 'workon' command

[timtalbot]

Problem Statement

Currently, the ptd workon command (cmd/workon.go) supports interactive shell sessions for workloads and control rooms with their respective Pulumi steps (bootstrap, persistent, etc.), but does not support custom steps defined through the custom steps framework.

Developers working on custom steps need to manually set up their environment and manage Pulumi stacks, which is inconsistent with the streamlined workflow available for standard steps.

Current Behavior

• ✅ ptd workon my-workload bootstrap - Works for standard steps
• ✅ ptd workon my-control-room persistent - Works for standard steps
• ❌ ptd workon my-workload my-custom-step - Not supported for custom steps
• ❌ ptd workon my-control-room monitoring-dashboards - Not supported for custom steps

Proposed Solution

Extend the workon command to recognize and support custom steps defined in the target's manifest file, providing the same interactive shell experience with proper Pulumi stack initialization.

Acceptance Criteria

• ptd workon successfully starts an interactive shell
• Pulumi stack is properly initialized/selected for the custom step
• Environment variables are correctly set for custom step context
• Custom step's Go module dependencies are accessible
• Command works for both control room and workload custom steps