Extract SupportedShells constant to constants package (gt-4u682)

- Added constants.SupportedShells for consistent shell list
- Updated 7 usages across start.go, crew_lifecycle.go, crew_helpers.go, tmux.go
- All tests pass

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/constants/constants.go

@@ -101,6 +101,10 @@ const (
 	RoleDeacon = "deacon"
 )
 
+// SupportedShells lists shell binaries that Gas Town can detect and work with.
+// Used to identify if a tmux pane is at a shell prompt vs running a command.
+var SupportedShells = []string{"bash", "zsh", "sh", "fish", "tcsh", "ksh"}
+
 // Path helpers construct common paths.
 
 // MayorRigsPath returns the path to rigs.json within a town root.