Improve development sandbox "manual" mode

_Generated by an AI assistant on behalf of @rpbritton._

### Problem statement

Raised as an action item in the Charm QA Triage Retrospective (9 Sept 2026). The development sandbox's manual mode (for direct, hands-on interaction rather than fully automated/AI-driven flows) needs improvement, based on friction reported during the retrospective around environment setup/troubleshooting time and scope management.

### Enhancement Proposal

Improve the sandbox's manual mode so a developer can more easily drive it step by step (setup, deploy, validate, teardown) without relying on the AI assistant to construct correct command sequences each time.

### What needs to get done?

- Identify current pain points in manual mode usage (setup, substrate selection, bundle deploy, validator run).
- Improve or document the manual workflow to close those gaps.

Start by reviewing the existing manual-mode workflow for setup, substrate selection, bundle deployment, validator execution, and teardown. Identify the reported friction points and decide whether each gap needs workflow improvements or documentation; done means a developer can drive these steps without relying on the AI assistant to construct command sequences.

Written by the indexing model from the issue text.