docs: add code-level context compaction evolution record (v1→v2→correctness)

[Copilot]

This change adds a detailed design document for the upstream context compaction evolution, covering the full path from initial compaction orchestration to v2 iterative summarization and later correctness hardening. It consolidates system-level milestones and code-level deltas into one scan-friendly reference under docs/design/.

• What this adds

  • New document: docs/design/context-compaction-evolution.md
  • End-to-end evolution timeline: v1 baseline → PR perf/context compaction AgentFlocks/flocks#147 observability upgrade → PR feat/context compaction v2 AgentFlocks/flocks#321 v2 architecture → PR fix(compaction): account for post-call tool results AgentFlocks/flocks#699 correctness fix (+ key follow-up refinements)

• Code-level change mapping

  • Enumerates modules, classes, and method signatures involved in each phase (e.g., compaction orchestrator/process paths, iterative summary APIs, pruning/token-estimation paths)
  • Captures algorithmic changes (iterative summary state + rebuild interval, overflow policy, pre/post prune, delta-only compaction, post-call tool-result accounting)

• Protocol + policy coverage

  • Documents frontend/backend protocol updates (notably session.compaction_progress SSE stages and FE action handling)
  • Summarizes compaction policy/config shifts (tiering, thresholds, tool-retention strategy, cooldown/anti-thrashing mechanics)

• Impact and compatibility analysis

  • Includes per-phase touched files and churn stats (files/additions/deletions)
  • Notes backward/forward compatibility implications for each systemic upgrade

## 4. 阶段 C：PR #321（v2 机制升级）
#### 4.2.1 迭代式摘要（E1/E2）
- summary.py: build_iterative_prompt(...)
- summary.py: summarize_chunked_iterative(...)
- models.py: DEFAULT_COMPACTION_PROMPT_WITH_PREVIOUS
- models.py: ITERATIVE_SUMMARY_REBUILD_INTERVAL = 5