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/tmux/tmux.go

@@ -9,6 +9,8 @@ import (
 	"os/exec"
 	"strings"
 	"time"
+
+	"github.com/steveyegge/gastown/internal/constants"
 )
 
 // Common errors
@@ -433,7 +435,7 @@ func (t *Tmux) WaitForCommand(session string, excludeCommands []string, timeout
 // WaitForShellReady polls until the pane is running a shell command.
 // Useful for waiting until a process has exited and returned to shell.
 func (t *Tmux) WaitForShellReady(session string, timeout time.Duration) error {
-	shells := []string{"bash", "zsh", "sh", "fish", "tcsh", "ksh"}
+	shells := constants.SupportedShells
 	deadline := time.Now().Add(timeout)
 	for time.Now().Before(deadline) {
 		cmd, err := t.GetPaneCommand(session)