[FEATURE]: Per-agent compaction config (disable compaction for specific agents) #16375

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Describe the enhancement you want to request

Problem

Compaction is currently only configurable globally via compaction: { auto, prune, reserved }. There is no way to control compaction behavior per-agent.

This causes issues with read-only agents like explore -- when compaction triggers during an explore subagent session, the agent loses its read-only constraints and starts writing code.

Reproduction

1. Configure the explore subagent with a codex model (e.g. opencode/codex-spark)
2. Use explore for codebase exploration until context gets long enough to trigger compaction
3. After compaction, explore starts making file edits despite being a read-only agent

Proposed solution

Allow compaction to be configured per-agent, consistent with how tools, permission, model, steps, and temperature already support per-agent overrides.

JSON config example:

{
  "agent": {
    "explore": {
      "compaction": {
        "auto": false
      }
    }
  }
}

Markdown agent example:

---
description: Fast codebase exploration
mode: subagent
compaction:
  auto: false
---

Alternative (minimum fix)

If per-agent compaction config is too broad, at minimum the compaction agent should preserve the invoking agent's tool/permission constraints after compacting. If an agent has write: false and edit: false, those constraints should survive compaction.

Why this makes sense

• The explore agent is documented as "A fast, read-only agent for exploring codebases. Cannot modify files." (docs). Compaction breaking this contract is unexpected.
• Per-agent config already exists for: model, prompt, temperature, top_p, steps, tools, permission, mode, hidden, color, disable. Adding compaction is consistent with this pattern.
• Related (but different scope): Experimental compaction improvements (prefix preservation + prompt customization) #11497 (compaction prompt customization), Feature Request: Configurable context limit and auto-compaction threshold #8140 (global compaction threshold).