context related issue

[vigo999]

1. /clear doesn't actually clear context — it only clears the screen. The LLM still sees full history. This is likely a bug or at
   least confusing to users.
2. Token estimation is heuristic only (4 chars/token) — can be 20-40% off, meaning compaction triggers at wrong times for a 200K
   window.
3. "Summarize" compaction strategy is fake — just generates a template string counting messages, no actual LLM summarization.
4. No /sessions command — users can resume/replay from CLI args but can't list or manage sessions from inside the app.
5. Snapshot only saved on /compact — if the app crashes, snapshot may be stale. Should auto-save periodically.
6. No context usage visibility — users don't see how much of the 200K window is used until they /compact. The infrastructure exists
   (emitTokenUsageSnapshot) but isn't called regularly.
7. No session cleanup — ~/.mscli/sessions/ will grow unbounded over time.

[zhangfeiran]

resolves in #63, #64, and #70

[zhangfeiran]

TODO：

• cc的其他本地ctx管理策略：micro-compact（自动清理1h+5条前的tool result）、CONTEXT_COLLAPSE（折叠旧消息）、HISTORY_SNIP（旧消息排序），实测cc+kimi并未触发，待后续捋清逻辑后引入
• cc中esc+esc的“time machine"恢复先前对话checkpoint的功能