SSH persistent shell now defaults to true — non-local backends benefit
most from state persistence across execute() calls. Local backend
remains opt-in via TERMINAL_LOCAL_PERSISTENT env var.
New config.yaml option: terminal.persistent_shell (default: true)
Controls the default for non-local backends. Users can disable with:
hermes config set terminal.persistent_shell false
Precedence: per-backend env var > TERMINAL_PERSISTENT_SHELL > default.
Wired through cli.py, gateway/run.py, and hermes_cli/config.py so the
config.yaml value reaches terminal_tool via env var bridge.
