Make herdr persistent by default.
Launching herdr now starts or reattaches to a background session
server. Clients can detach and reattach while panes and agent
processes keep running. Session mode now supports multi-client attach,
auto-detect startup, and a thin-client/headless-server split.
This also refactors the large app, ui, input, pane, config,
workspace, and persistence modules into smaller focused submodules,
while preserving behavior and colocating tests with the code they
exercise.
Upgrade notes:
- persistence mode is now the default
- in-app quit detaches the current client instead of stopping the server
- use `herdr server stop` to stop the background session
- use `--no-session` for the old single-process behavior
- default socket paths now live under the config directory
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