docs(skill): add software-engineering README and same-dir reference test

SKILL.md header pointed at same-dir README.md but the pack had no
such file (CHANGELOG was supplied by #217, README was not). This
track adds the README only: purpose, chapter map, appendix
provenance entry point, seeding behaviour, and known boundaries.
It states no rule, changes no SKILL.md semantics, and does not
duplicate the CHANGELOG/script explanations -- overlapping parts
are pointers, with an explicit division-of-labour section.
Deletion retention is written only as the tombstone-design
placeholder, deferring to the skill_seed implementation, because
no tombstone implementation has landed. Test pins that every
file SKILL.md cites as same-directory exists (CHANGELOG, README).

cloud/tests/test_skill_readme_refs.py

@@ -0,0 +1,88 @@
+"""Same-directory references of the software-engineering skill.
+
+Background: SKILL.md's header points at two same-directory files --
+``CHANGELOG.md`` ("see CHANGELOG") and ``README.md`` (install path).
+CHANGELOG was supplied by the appendix-provenance track; README was
+still missing, so the header's install-path pointer dangled. This file
+pins the repair, and nothing else:
+
+- every file SKILL.md cites with the qualifier "same directory"
+  (``同目录 `X.md` ``) exists in the skill directory -- today exactly
+  CHANGELOG.md and README.md, so a future dangling same-directory
+  reference fails here instead of shipping silently;
+- README covers the topics it is supposed to cover (package layout,
+  appendix provenance entry point, seeding behaviour, and the
+  tombstone-design placeholder for deletion retention) without
+  becoming a second copy of the appendix or of CHANGELOG: it must
+  not contain appendix chapter headings, and it must not invent a
+  tombstone marker filename.
+
+Scope guard: this test never edits SKILL.md. In particular it does
+NOT assert on the templates/artifacts/ names mentioned in the SKILL
+body (PRD outline, intake questions, acceptance checklist): those are
+not same-directory references, they are not in this pack, and their
+status is README's stated known boundary, not a dangling header
+reference to pin here.
+"""
+
+import re
+import unittest
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+SKILL_DIR = REPO_ROOT / "cloud" / "skills" / "software-engineering"
+SKILL_FILE = SKILL_DIR / "SKILL.md"
+README_FILE = SKILL_DIR / "README.md"
+
+# ``同目录 `X.md` `` -- the header's same-directory citations only.
+SAME_DIR_REF_RE = re.compile(r"同目录\s*`([^`]+\.md)`")
+
+
+class SkillReadmeRefsTestCase(unittest.TestCase):
+    def test_same_dir_refs_all_exist(self):
+        skill_text = SKILL_FILE.read_text(encoding="utf-8")
+        refs = set(SAME_DIR_REF_RE.findall(skill_text))
+        self.assertEqual(
+            refs, {"CHANGELOG.md", "README.md"},
+            "SKILL.md 的同目录引用应恰为 CHANGELOG 与 README，实际：%s"
+            % sorted(refs))
+        for name in sorted(refs):
+            self.assertTrue(
+                (SKILL_DIR / name).is_file(),
+                "SKILL.md 引用的同目录文件缺失：%s" % name)
+
+    def test_readme_covers_required_topics(self):
+        self.assertTrue(README_FILE.is_file(), "README.md 缺失")
+        text = README_FILE.read_text(encoding="utf-8")
+        for fragment in (
+                "templates/roles/",
+                "check_appendix.py",
+                "CHANGELOG.md",
+                "skill_seed",
+                "只复制各技能的 `SKILL.md` 一个文件",
+                # Deletion retention is NOT implemented: README must
+                # say so via the tombstone-design placeholder, and
+                # defer to the skill_seed implementation, verbatim.
+                "删除保留机制见 tombstone 设计",
+                "行为以 skill_seed 实现为准",
+                "分工",
+        ):
+            self.assertIn(fragment, text, "README.md 应提及：%s" % fragment)
+
+    def test_readme_does_not_duplicate_appendix(self):
+        text = README_FILE.read_text(encoding="utf-8")
+        self.assertIsNone(
+            re.search(r"^## 附录角色：", text, flags=re.M),
+            "README 不应复制附录章节标题，否则会成为第二份会漂移的附录")
+
+    def test_readme_invents_no_tombstone_marker(self):
+        text = README_FILE.read_text(encoding="utf-8")
+        self.assertIn("tombstone", text, "README 应提及 tombstone 设计占位")
+        self.assertIsNone(
+            re.search(r"tombstone\.[a-z]+", text),
+            "tombstone 未落地为实现，README 不得编造标记文件名")
+        self.assertNotIn(".tombstone", text)
+
+
+if __name__ == "__main__":
+    unittest.main()