Add WaitForShellReady call before SendKeys in all agent managers
(deacon, mayor, witness, refinery). This prevents intermittent
"can't find pane" errors that occur when the tmux session is
created but the shell isn't ready to receive input yet.
The issue manifests under load (e.g., during `gt up` when multiple
agents start in sequence) where the 200ms delay in SendKeysDelayed
isn't sufficient for the pane to be fully initialized.
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