Memory autosave: digest finished turns into the memory store automatically

[A-x6]

The memory engine (#29 added the store, save/recall tools, and TUI commands) currently only captures what the model explicitly saves via the memory_save tool. The capture pipeline in packages/memory (MemoryCapture.turn, MemoryTurn.open/close) supports automatic turn-close digestion, but nothing in the CLI host drives it — so autosave never runs.

Wire the host side:

• Implement the capture ports against the CLI session store and provider (MemoryPorts.SessionPort reading the latest finished turn + diffs, MemoryPorts.ModelPort resolving/running the consolidation model via the ai SDK).
• Open a memory turn when a prompt loop starts and close it when the loop exits (completed / error / interrupted), so the package can digest the turn or schedule an idle flush.
• The whole path must be a cheap no-op when memory is disabled, and must never break or delay the session loop (fire-and-forget close, errors swallowed and logged).