feat: 新增 git retime，把指定日期范围的提交时间改到 21:00-23:00 并强推

- 保留每个提交的原日期，只重排时分秒到 21:00-23:00；同日严格递增，
  间隔 5-40 分钟且秒位随机；窗口不足时自动压紧并在预览中标注
- 支持 --window/--since/--until、--interval（含 match-remote）、--seed、
  --remote、--branch、--no-push
- 安全措施：默认只预览（退出码 2）、脏工作区拒绝、改写前创建
  refs/kopsc-backup/<分支>-<时间戳> 备份、推送前比对 tree 哈希、
  只用 --force-with-lease
- BuiltinRequest 增加 Confirmed，使内置命令能获知 CLI 已确认 --yes
- 文档：新增 README.git.md，主 README 补充导航与示例

internal/executor/gitrewrite_test.go

@@ -0,0 +1,122 @@
+package executor
+
+import (
+	"context"
+	"strings"
+	"testing"
+	"time"
+)
+
+// rewriteFixture builds a fake git whose two commits c1 -> c2 live on main.
+func rewriteFixture() *fakeGitRunner {
+	meta := func(hash, tree, parents string) map[string]gitResponse {
+		return map[string]gitResponse{
+			"log -1 --format=" + rawCommitFormat + " " + hash: {
+				out: strings.Join([]string{hash, tree, parents, "Odboy", "tianjun@odboy.cn", "Odboy", "tianjun@odboy.cn", "subject of " + hash}, "\t") + "\n",
+			},
+			"log -1 --format=%B " + hash: {out: "subject of " + hash + "\n\nbody line\n"},
+		}
+	}
+
+	responses := map[string]gitResponse{}
+	for _, entry := range []map[string]gitResponse{meta("c1", "tree1", "base"), meta("c2", "tree2", "c1")} {
+		for k, v := range entry {
+			responses[k] = v
+		}
+	}
+	// New commits are named deterministically by the tree they carry.
+	responses["commit-tree tree1 -p base -m subject of c1\n\nbody line"] = gitResponse{out: "n1\n"}
+	responses["commit-tree tree2 -p n1 -m subject of c2\n\nbody line"] = gitResponse{out: "n2\n"}
+	return &fakeGitRunner{responses: responses}
+}
+
+func TestRewriteChainRebuildsInOrder(t *testing.T) {
+	runner := rewriteFixture()
+	plan := retimePlan{
+		Commits: []retime{
+			{Hash: "c1", Short: "c1", NewTime: time.Date(2026, 10, 1, 21, 7, 43, 0, cst)},
+			{Hash: "c2", Short: "c2", NewTime: time.Date(2026, 10, 1, 21, 32, 11, 0, cst)},
+		},
+		Branch: "main",
+		Remote: "origin",
+	}
+
+	tip, rebuilt, err := rewriteChain(context.Background(), runner, plan)
+	if err != nil {
+		t.Fatalf("rewriteChain: %v", err)
+	}
+	if tip != "n2" {
+		t.Errorf("tip = %q, want n2", tip)
+	}
+	if len(rebuilt) != 2 || rebuilt[0].New != "n1" || rebuilt[1].New != "n2" {
+		t.Fatalf("rebuilt = %+v", rebuilt)
+	}
+
+	// The dates must be passed to git exactly as planned, with the original
+	// identities kept.
+	if len(runner.envs) != 2 {
+		t.Fatalf("expected env for two commits, got %d", len(runner.envs))
+	}
+	joined := strings.Join(runner.envs[0], " ")
+	for _, want := range []string{
+		"GIT_AUTHOR_NAME=Odboy",
+		"GIT_AUTHOR_EMAIL=tianjun@odboy.cn",
+		"GIT_AUTHOR_DATE=2026-10-01T21:07:43+08:00",
+		"GIT_COMMITTER_DATE=2026-10-01T21:07:43+08:00",
+	} {
+		if !strings.Contains(joined, want) {
+			t.Errorf("first commit env missing %q: %v", want, runner.envs[0])
+		}
+	}
+	if !strings.Contains(strings.Join(runner.envs[1], " "), "GIT_AUTHOR_DATE=2026-10-01T21:32:11+08:00") {
+		t.Errorf("second commit env = %v", runner.envs[1])
+	}
+}
+
+func TestRebuildCommitKeepsTreeParentsAndMessage(t *testing.T) {
+	var gotArgs []string
+	var gotEnv []string
+	runner := &fakeGitRunner{
+		responses: map[string]gitResponse{
+			"commit-tree T -p P1 -p P2 -m msg line 1\nline 2": {out: "newhash\n"},
+		},
+	}
+	orig := runner.RunEnv
+	_ = orig
+
+	meta := commitMeta{
+		Tree: "T", Parents: []string{"P1", "P2"},
+		AuthorName: "A", AuthorEmail: "a@e", CommitName: "C", CommitEmail: "c@e",
+		Body: "msg line 1\nline 2",
+	}
+	hash, err := rebuildCommit(context.Background(), runner, meta, time.Date(2026, 10, 1, 21, 0, 0, 0, cst))
+	if err != nil {
+		t.Fatalf("rebuildCommit: %v", err)
+	}
+	if hash != "newhash" {
+		t.Errorf("hash = %q", hash)
+	}
+	gotArgs = append(gotArgs, runner.calls...)
+	gotEnv = append(gotEnv, strings.Join(runner.envs[0], " "))
+	if !strings.Contains(gotEnv[0], "GIT_COMMITTER_EMAIL=c@e") {
+		t.Errorf("env = %v", gotEnv)
+	}
+}
+
+func TestMoveBranch(t *testing.T) {
+	runner := &fakeGitRunner{responses: map[string]gitResponse{
+		"update-ref refs/heads/main n2": {out: ""},
+	}}
+	if err := moveBranch(context.Background(), runner, "main", "n2"); err != nil {
+		t.Fatalf("moveBranch: %v", err)
+	}
+	if len(runner.calls) != 1 || runner.calls[0] != "update-ref refs/heads/main n2" {
+		t.Fatalf("calls = %v", runner.calls)
+	}
+}
+
+func TestRewriteChainEmptyPlan(t *testing.T) {
+	if _, _, err := rewriteChain(context.Background(), rewriteFixture(), retimePlan{}); err == nil {
+		t.Fatal("expected an error for an empty plan")
+	}
+}