Agent.Start and ConnectionManager.Start now take a context.Context. The
manager runs until that context is cancelled or the process receives
SIGINT/SIGTERM. On shutdown it waits for in-flight reconnect attempts
before closing connections.
The reconnect delay, WebSocket dial and pending handshake now stop when
the context is cancelled. A context-aware dialer keeps proxy selection
from the environment. WebSocket callbacks no longer block on eventChan
after the event loop exits. A late reconnect can no longer restart the
SSH server once shutdown begins.
BREAKING CHANGE: agent.Agent.Start now requires a context.Context.
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Co-authored-by: hank <hank@henrygd.me>
