[Codex] Scoped context compaction for exploratory phases

### What variant of Codex are you using? CLI ### What feature would you like to see? A way to compress a **specific segment** of the conversation history — not the whole thing — when that segment represents a completed exploratory phase. ### Problem Current context management in Codex compresses the entire history when approaching limits. But real agentic sessions have structure: ``` turns 1–30: initial setup, requirements gathering turns 31–80: exploring a bug → lots of dead ends, tool calls, false starts turn 81: conclusion reached: "the issue is in X, fix is Y" turns 82–...: implementing the fix ``` When compaction fires, the entire history gets summarized — including turns 1–30 (still relevant setup context) and turns 31–80 (completed exploration). The precise conclusion from turn 81 gets diluted into the global summary, and the setup context from turns 1–30 is unnecessarily compressed too. ### Proposed solution A **scoped compaction** primitive: compress only a d