The quick-add command (used by shell hook's "Add to Gas Town?" prompt)
previously only checked hardcoded paths ~/gt and ~/gastown, ignoring
GT_TOWN_ROOT and any other Gas Town installations.
This caused rigs to be added to the wrong town when users had multiple
Gas Town installations (e.g., ~/gt and ~/Documents/code/gt).
Fix the town discovery order:
1. GT_TOWN_ROOT env var (explicit user preference)
2. workspace.FindFromCwd() (supports multiple installations)
3. Fall back to ~/gt and ~/gastown
