feat: AutoZero 通用 Agent 框架（从 AutoGo 五阶段拆分完成）

llm（三协议 Provider）/ session（JSONL 持久化）/ tools（Registry、围栏、
builtin、computer）/ cdp（browser）/ skills / mcp / agent（循环、两级压缩、
编排、审批门控）+ 底座 jsonl/textx/execx/diag/scratch/spill/imagecache。

产品装配代码（config 桥接、schedule 工具、memory/plan/retro/prompt）
留在 AutoGo，宿主接入方式见 MIGRATION.md。

agent/checkpoint.go

@@ -0,0 +1,130 @@
+package agent
+
+import (
+	"fmt"
+	"strings"
+
+	"git.gopher.ink/mirrors2/AutoZero/llm"
+	"git.gopher.ink/mirrors2/AutoZero/session"
+)
+
+const (
+	checkpointPrefix = "Here is a summary of the conversation so far:\n\n"
+	checkpointSuffix = "\n\nContinue the task based on this summary."
+)
+
+// checkpointMessage 把交接文档装成工作集里的第一条 user 消息。
+// 形状保持不变（测试与旧会话缓存都依赖这个前缀）。
+func checkpointMessage(text string) llm.Message {
+	return llm.Message{Role: llm.RoleUser, Content: checkpointPrefix + text + checkpointSuffix}
+}
+
+func isCheckpointMessage(m llm.Message) bool {
+	return m.Role == llm.RoleUser && strings.HasPrefix(m.Content, checkpointPrefix)
+}
+
+// LoadCheckpoint 从 sidecar 恢复压缩交接文档，并把工作集重建成
+// [checkpoint] + transcript[Covered:]。覆盖边界用消息指纹校验：历史被回退或
+// 改写后指纹对不上，就当作没有 checkpoint——绝不能拿覆盖了别的消息的旧摘要继续跑。
+func (a *Agent) LoadCheckpoint() bool {
+	if a.Session == nil || a.Session.Path == "" {
+		return false
+	}
+	cp, ok := session.LoadCheckpoint(a.sessionID())
+	if !ok || cp.Covered <= 0 || cp.Covered > len(a.Session.Messages) {
+		return false
+	}
+	if cp.TailHash != session.MessageFingerprint(a.Session.Messages[cp.Covered-1]) {
+		return false
+	}
+	a.compactMu.Lock()
+	defer a.compactMu.Unlock()
+	a.checkpoint = cp.Text
+	a.checkpointCovered = cp.Covered
+	a.checkpointHash = cp.TailHash
+	a.compacted = []llm.Message{checkpointMessage(cp.Text)}
+	a.compactFrom = cp.Covered
+	a.compactGaveUp = false
+	a.resetInputUsage()
+	return true
+}
+
+// validateCheckpointLocked 在 transcript 被回退/改写后作废旧 checkpoint。
+// 覆盖边界的指纹对不上就整体退回完整历史，绝不拿一份覆盖了别的消息的摘要继续跑。
+// 调用方持 compactMu。
+func (a *Agent) validateCheckpointLocked() {
+	if a.checkpoint == "" || a.Session == nil {
+		return
+	}
+	valid := a.checkpointCovered > 0 && a.checkpointCovered <= len(a.Session.Messages) &&
+		a.checkpointHash == session.MessageFingerprint(a.Session.Messages[a.checkpointCovered-1])
+	if valid {
+		return
+	}
+	a.checkpoint = ""
+	a.checkpointCovered = 0
+	a.checkpointHash = ""
+	a.compacted = nil
+	a.compactFrom = 0
+	a.compactGaveUp = false
+}
+
+// saveCheckpointLocked 把当前 checkpoint 写进 sidecar。调用方持 compactMu。
+func (a *Agent) saveCheckpointLocked() {
+	id := a.sessionID()
+	if id == "" || a.checkpoint == "" {
+		return
+	}
+	err := session.SaveCheckpoint(id, session.Checkpoint{
+		Text:     a.checkpoint,
+		Covered:  a.checkpointCovered,
+		TailHash: a.checkpointHash,
+	})
+	if err != nil {
+		fmt.Fprintf(a.Opt.Stderr, "[context] save checkpoint failed: %v\n", err)
+	}
+}
+
+// resetInputUsage 让下一次压缩判据退回估算：视图刚被替换，旧请求的真实 token
+// 数已经不对应当前工作集了。
+func (a *Agent) resetInputUsage() {
+	a.usageMu.Lock()
+	a.lastInputTokens = 0
+	a.lastInputMsgs = 0
+	a.lastSystemBytes = 0
+	a.usageMu.Unlock()
+}
+
+// noteRequestUsage 记下一次成功请求的真实 prompt token（含缓存命中/写入）。
+// 下一个 step 的压缩判据用它 + 新增消息估算，而不是纯 bytes/3。
+func (a *Agent) noteRequestUsage(u *llm.Usage, view []llm.Message) {
+	if u == nil || u.InputTokens <= 0 {
+		return
+	}
+	sysLen := len(a.sysNow())
+	a.usageMu.Lock()
+	a.lastInputTokens = u.InputTokens
+	a.lastInputMsgs = len(view)
+	a.lastSystemBytes = sysLen
+	a.usageMu.Unlock()
+}
+
+// observedTokens 返回当前工作集的 token 观测值：有真实请求数据时以它为基准，
+// 加上那次请求之后新增消息与 system 动态段的估算；没有就退回 bytes/3。
+func (a *Agent) observedTokens(view []llm.Message) int {
+	a.usageMu.Lock()
+	last, lastN, lastSys := a.lastInputTokens, a.lastInputMsgs, a.lastSystemBytes
+	a.usageMu.Unlock()
+	sys := a.sysNow()
+	if last <= 0 || lastN < 0 || lastN > len(view) {
+		return estimateTokens(view, sys)
+	}
+	added := 0
+	if lastN < len(view) {
+		added = estimateTokens(view[lastN:], "")
+	}
+	if delta := len(sys) - lastSys; delta > 0 {
+		added += delta / 3
+	}
+	return last + added
+}