Fix tmux error handling for "no current target" (#755)

When starting crew without mayor running, tmux has-session can return
"no current target" if no tmux server exists. This error was not
mapped to ErrNoServer, causing crew start to fail instead of
bootstrapping a new tmux server.

Add "no current target" to the ErrNoServer detection so crew (and
other agents) can start independently without requiring an existing
tmux session.

Reproduction:
- Ensure no tmux server running (tmux kill-server)
- Run: gt crew start <rig>/<name>
- Before fix: "Error: checking session: tmux has-session: no current target"
- After fix: Crew session starts successfully

Co-authored-by: Shaun <TechnicallyShaun@users.noreply.github.com>

internal/tmux/tmux.go

@@ -63,7 +63,8 @@ func (t *Tmux) wrapError(err error, stderr string, args []string) error {
 
 	// Detect specific error types
 	if strings.Contains(stderr, "no server running") ||
-		strings.Contains(stderr, "error connecting to") {
+		strings.Contains(stderr, "error connecting to") ||
+		strings.Contains(stderr, "no current target") {
 		return ErrNoServer
 	}
 	if strings.Contains(stderr, "duplicate session") {