[Feature] 设置页增加上下文压缩阈值（keepRecent / reserve） / Expose compaction keepRecentTokens and reserveTokens in Settings

[xingleiwu]

Problem / 问题

Automatic context compaction is configurable in storage:

"contextCompaction": {
  "enabled": true,
  "keepRecentTokens": 20000,
  "reserveTokens": 16384
}

Sidecar / host-core read keepRecentTokens and reserveTokens. Settings UI has no controls for them (no matches in the settings renderer). Long threads compact on defaults only; changing the threshold means editing pi.sqlite.

自动上下文压缩的阈值已经在 sqlite 里（keepRecentTokens / reserveTokens / enabled），sidecar 会读。设置页没有入口，长对话何时压缩、压完留多少只能改库或吃默认。

Proposed change / 期望改动

Add a Context compaction section (Agent settings or Models — either is fine):

• Toggle: enable automatic compaction (enabled)
• Keep recent: presets 20k / 32k / 64k / 100k, plus a number field (keepRecentTokens)
• Reserve for output: presets 8k / 16k / 32k / 64k (reserveTokens)
• One-line help: when the window is nearly full, a checkpoint is taken; keep-recent is how much recent context survives the summary; reserve is space left for this turn's output

设置里增加「上下文压缩」：开关、保留最近、为输出预留，并配常用档位 + 可手改数字。说明：窗口快满时做检查点；保留最近 = 摘要后仍留给模型的近期 token；预留 = 压缩后给本轮输出留的空位。

Alternatives / 其他方案

Leave it sqlite-only: users cannot tune it. Deriving both numbers only from the model window: too coarse (20k keep + 16k reserve is wrong for both 128k and 1M).

Additional context / 补充信息

PI-Desktop 0.14.5 · Windows. Related ADRs: 0030, 0049, 0061. This is the missing settings surface, not a change to the compaction algorithm.

[ZxlDragonDoctor]

I'd like to take this. I'll add a Context compaction section in Settings (enabled toggle + keepRecentTokens / reserveTokens presets and number fields), wire it to the existing contextCompaction storage keys the sidecar already reads, and cover with typecheck/tests where the settings surface allows.

[ZxlDragonDoctor]

Withdrawing my claim after reading the accepted ADRs.

ADR 0061 clause 9 and ADR 0064 explicitly keep Settings free of compaction controls and ignore persisted contextCompaction values (deliberately: a user who once turned compaction off would otherwise have no switch left). Budgets are derived from the model window (keepRecentTokens = clamp(hardLimit * 0.2, 8k, 64k)), not from fixed 20k/16k knobs.

Re-adding the Settings surface would reverse that decision. Happy to implement if maintainers reopen the product question and amend the ADR; otherwise this issue may need a won't-fix / design note.