[FEATURE]: per-model compaction threshold (reserved/buffer)

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Describe the enhancement you want to request

Summary

Allow configuring compaction thresholds per model instead of a single global value.

Currently the top-level compaction block (auto, prune, tail_turns, preserve_recent_tokens, reserved) is global — one value applied to every model/agent. There is no per-model or per-agent compaction field in the schema.

Use case

Users switch between models with very different context windows (e.g. a 32k local model and a 128k+ frontier model). A single reserved token buffer means the effective trigger point scales linearly with the model's context limit, which is not what we want: we want to compact a small model earlier (relative to its window) and a large model later, or vice-versa — independent reserved/buffer per model.

Current workaround (and its limitation)

The effective trigger is approximately model.limit.context − reserved. By setting a per-model limit.context in provider.<name>.models.<id>, the trigger point can be shifted per model. However, reserved (the headroom) stays a single global value, so you cannot set an independent buffer per model.

Proposed API

Something like a per-model override, e.g.:

{
  "provider": {
    "my-provider": {
      "models": {
        "small-model": {
          "limit": { "context": 32000, "output": 4096 },
          "compaction": { "auto": true, "reserved": 4000 }
        },
        "big-model": {
          "limit": { "context": 128000, "output": 8192 },
          "compaction": { "auto": true, "reserved": 24000 }
        }
      }
    }
  }
}

or a hook that receives the model and returns the compaction settings, so plugins can compute a threshold at runtime.

Benefits

• Predictable, model-aware compaction behavior for multi-model setups.
• No wasted context on large models or over-aggressive compaction on small models.
• Enables provider-level defaults with per-model overrides, consistent with how limit already works.