Agent now owns the full turn lifecycle: Submit, executeTurn, turnError,
autoCompactLimit, autoWarnLimit, spawnContext, and runCompact all live
on *Agent in turn.go.
Session.Submit is a thin dispatcher: slash commands are handled at
session level (they need sessionsDir, listHandlers, etc.), then
non-command input is delegated to Agent.Submit.
Session.IsRunning and Session.Interrupt delegate to Agent methods.
turnError is an overridable func field on Agent (default: hook-based
implementation in defaultTurnError). saveSession and flushSave are
injected as callbacks from Session into Agent at construction time.
This completes the agent/session ownership separation for the turn
execution path. The Agent is now a self-contained entity that can
run turns independently given its injected dependencies.
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