Fix apply --force against a dirty tree; clarify patch-count guard

Two issues found by exercising the scripts end to end:

- `git checkout -B` refuses to run when the worktree is dirty, so
  `make apply FORCE=1` aborted in exactly the situation FORCE exists
  to handle. Use `checkout -f -B`; the non-forced path has already
  verified the tree is clean, so forcing changes nothing there.

- The commit-count guard claimed the user forgot `make rebuild`, but
  deleting a patch file on purpose trips it identically. Say both,
  and document the FORCE=1 form in the README's "drop the example
  patches" instructions, which would otherwise have failed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -175,11 +175,12 @@ git format-patch --no-stat -N --zero-commit --full-index --no-signature
    （或 `)`）结尾——**改上游代码时顺手确认它的测试仍然成立**，这个补丁本身就是个例子。
 2. **`0002`** 新增文件——根目录 `FORK.md`。
 
-不需要就直接删：
+不需要就直接删。注意要带 `FORCE=1`——`work/` 里还留着这两个补丁的提交，而删掉补丁文件后
+`patches/` 比 `work/` 少，安全检查会拦下来（它无法区分「你故意删的」和「你忘了 rebuild」）：
 
 ```sh
 rm patches/0001-*.patch patches/0002-*.patch
-make apply
+make apply FORCE=1
 ```
 
 ## 许可

scripts/apply-patches.sh

@@ -38,8 +38,9 @@ else
       want="$(count_patches)"
       if [[ "$have" != "$want" ]]; then
         die "work/ has $have commit(s) above $BASE_TAG but patches/ has $want patch file(s).
-    You probably forgot to run 'make rebuild' after committing.
-    To discard the work/ commits and replay patches/ as-is: make apply FORCE=1"
+    Either you committed in work/ without running 'make rebuild' (export it:
+    make rebuild), or you added/removed patch files on purpose -- in which case
+    discard the work/ commits and replay patches/ as-is:  make apply FORCE=1"
       fi
     fi
   fi
@@ -58,7 +59,10 @@ gitdir="$(git rev-parse --git-dir)"
 [[ -d "$gitdir/rebase-apply" ]] && git am --abort >/dev/null 2>&1 || true
 
 info "Resetting work/ to upstream $(git rev-parse --short "$REV")"
-git checkout -q -B "$WORK_BRANCH" "$REV"
+# -f is required: without it checkout refuses to run against a dirty tree, which
+# is precisely the case FORCE=1 exists to handle. The tree is already known
+# clean on the non-forced path, so forcing costs nothing there.
+git checkout -q -f -B "$WORK_BRANCH" "$REV"
 git reset -q --hard "$REV"
 
 # -x removes ignored files too, which is what we want for a pristine tree --