feat: configurable merge gate (branch protection + directory isolation) and skill body/appendix checks

- cloud/merge_gate.py: pure gate; directory check reuses the v0.3
  git diff --name-only merge-base command; protection modes
  record (default, explicit confirm required when unprotected),
  enforce (always blocks when unprotected), off (explicit only).
  Confirm covers protection risk only, never directory violations.
  Protection status is caller/env-declared only; unknown is a
  recorded risk, not a claim of protection.
- server POST /tasks/{id}/merge: applies the gate only when gate
  evidence is supplied (or env declares protection); no-evidence
  calls behave as before plus a merge_gate field in the response.
  Blocked merges return 409 without changing status; confirmed
  unprotected merges write a merge_gate_risk audit entry.
- skill_seed.py: whole-directory seeding evaluated and rejected
  (cannot preserve tenant deletions; no benefit while the pack
  holds only SKILL.md); SKILL.md-only behaviour unchanged.
- software-engineering SKILL.md: gate paragraph added to the body
  and to the coordinator + client appendix sections (v0.2 text on
  this base; port to v0.3 text on rebase after #199 merges).
- tests: 16 new tests covering gate modes, directory reuse,
  body/appendix bidirectional assertions naming the missing side,
  SKILL.md-only seeding and tenant-edit preservation.

cloud/server.py

@@ -74,6 +74,7 @@ from cloud.peer_messages import ensure_followups_table as _peer_ensure_followups
 from cloud.peer_messages import peer_message_prefix as _peer_prefix
 from cloud.peer_messages import row_get as _row_get
 from cloud.marker_tools import parse_spawn_defaults as _parse_spawn_defaults
+from cloud.merge_gate import gate_from_request as _merge_gate_from_request
 from cloud.tree import DEFAULT_MAX_DEPTH as _TREE_DEFAULT_DEPTH
 from cloud.tree import DEFAULT_MAX_ENTRIES as _TREE_DEFAULT_ENTRIES
 from cloud.tree import build_tree as _build_tree
@@ -7614,6 +7615,18 @@ class CloudControlPlaneHandler(BaseHTTPRequestHandler):
         必须是项目 maintainer 以上（approve_merge，403）、任务
         必须是 completed（其它状态一律 409，含已 merged 的重复
         调用，避免二次合并混淆）。成功后写审计 merge_task。
+
+        合并门禁（cloud/merge_gate，补 v0.3 技能文本检查在服务端
+        缺失的缺口；v0.3 之前本接口只改状态，不查目录、不查保护）：
+        请求体可选字段 files_changed / owned_dir /
+        approved_exceptions / branch_protection / merge_gate_confirm
+        / merge_gate_mode；一个都不给时行为与旧接口一致（只在响应
+        里附带门禁结果，不阻断）。给了证据才按门禁判：越目录
+        （v0.3 口径）或未保护且无确认（record 模式）或未保护
+        （enforce 模式）时 409 且不改状态；未保护 + 确认时放行，
+        风险记入响应与 merge_gate_risk 审计。确认只覆盖保护风险，
+        绝不覆盖目录违规。保护状态 unknown 时记风险、不阻断——
+        本服务没有可信渠道核实 GitHub 保护，unknown 不等于已保护。
         """
         row = self._fetch_task(task_id)
         if row is None:
@@ -7631,6 +7644,21 @@ class CloudControlPlaneHandler(BaseHTTPRequestHandler):
                 "status": task["status"],
             })
             return
+        body, jerr = self._read_json()
+        if jerr:
+            self._send_json(400, {"error": jerr})
+            return
+        if body.get("files_changed") is not None and not isinstance(
+                body.get("files_changed"), list):
+            self._send_json(400, {"error": "files_changed must be a list"})
+            return
+        gate = _merge_gate_from_request(body)
+        if not gate["allowed"]:
+            self._send_json(409, {
+                "error": "merge gate blocked",
+                "merge_gate": gate,
+            })
+            return
         now = time.time()
         conn = self._db()
         try:
@@ -7649,6 +7677,11 @@ class CloudControlPlaneHandler(BaseHTTPRequestHandler):
             conn.close()
         with _auth_store(self.server.db_path) as store:
             store.audit(user_id, task["project_id"], "merge_task", task_id)
+            if gate["protection"] == "unprotected":
+                store.audit(user_id, task["project_id"],
+                            "merge_gate_risk",
+                            "%s: %s" % (task_id, "; ".join(gate["risks"])))
+        task["merge_gate"] = gate
         self._send_json(200, task)
 
     # -- 运行时审批队列 ------------------------------------------------