feat(memory): add idle daily auto summaries

[GTC2080]

Closes #2565

Summary

• adds opt-in agent.auto_memory and agent.auto_memory_idle settings
• refreshes a per-project daily-summary-YYYY-MM-DD memory after an idle chat window
• merges only new conversation messages into the existing daily summary, keeping the current prompt prefix stable until the next session

Evidence

• No UI change; screenshots are not applicable.
• Validation: D:\Go\go\bin\go.exe test ./internal/agent ./internal/control ./internal/config ./internal/boot
• Validation: D:\Go\go\bin\go.exe test ./...

[esengine]

The engineering here is careful — the idle timer, the cursor-tracked region, the guards around in-flight turns are all well done, and it's gated off by default.

I'm going to pass on it for product fit rather than code. The whole job of this feature is to fire a background summarization call on an idle timer, and that call is a cache-miss by nature (fresh transcript + fresh system prompt each time). "Cheap, cache-first" is the load-bearing claim for this project, so a recurring ambient token spend — even opt-in — points users at exactly the path we tell them we avoid, and adds a non-core surface to maintain.

If we do ambient/long-term memory later, I'd want it as an explicit, user-invoked action (a slash command triggered when the user wants it), not an idle daemon. Closing this on that basis — not a reflection on the implementation, which is good.