fix(skills): address Copilot review nits (safety + UI guards)

backend/app/api/gateways.py

@@ -176,6 +176,9 @@ async def delete_gateway(
         await service.clear_agent_foreign_keys(agent_id=agent.id)
         await session.delete(agent)
 
+    # NOTE: The migration declares `ondelete="CASCADE"` for gateway_installed_skills.gateway_id,
+    # but some backends/test environments (e.g. SQLite without FK pragma) may not
+    # enforce cascades. Delete rows explicitly to guarantee cleanup semantics.
     installed_skills = await GatewayInstalledSkill.objects.filter_by(
         gateway_id=gateway.id,
     ).all(session)

backend/app/api/skills_marketplace.py

@@ -243,6 +243,15 @@ def _collect_pack_skills_from_index(
     if not index_file.is_file():
         return None
 
+    try:
+        stat = index_file.stat()
+    except OSError as exc:
+        raise RuntimeError("unable to read skills_index.json") from exc
+
+    # Hard cap to avoid loading an arbitrarily large file into memory.
+    if stat.st_size > 256 * 1024:
+        raise RuntimeError("skills_index.json is too large")
+
     try:
         payload = json.loads(index_file.read_text(encoding="utf-8"))
     except OSError as exc:
@@ -382,7 +391,10 @@ def _collect_pack_skills(source_url: str) -> list[PackSkillCandidate]:
             raise RuntimeError("timed out cloning pack repository") from exc
         except subprocess.CalledProcessError as exc:
             stderr = (exc.stderr or "").strip()
-            detail = stderr or "unable to clone pack repository"
+            # Avoid reflecting arbitrary stderr to callers; keep details minimal.
+            detail = "unable to clone pack repository"
+            if stderr:
+                detail = f"{detail}: {stderr.splitlines()[0][:200]}"
             raise RuntimeError(detail) from exc
 
         try:
@@ -397,6 +409,9 @@ def _collect_pack_skills(source_url: str) -> list[PackSkillCandidate]:
             branch = "main"
 
         branch = branch or "main"
+        # Fail closed on weird output to avoid constructing malformed URLs.
+        if any(ch in branch for ch in {"\n", "\r", "\t"}):
+            branch = "main"
 
         return _collect_pack_skills_from_repo(
             repo_dir=repo_dir,