fix(skill-versioning): consume divergence_tolerance, replace vacuous catalog-isolation guard

- divergence_tolerance was loaded from config but diff_flow_versions
  never consulted it, so structural_divergence was always plain
  sequence-equality regardless of the configured tolerance. Now
  actually applied per design.md D4.
- The skill-catalog-subscription isolation guard test asserted
  'skills.catalog' not in sys.modules, which is vacuously true since
  that module doesn't exist anywhere yet. Replaced with a real check
  against store.py's actual imports.

openspec: skill-versioning capability, archived change skill-learning-runtime

tests/test_skill_store.py

@@ -1,7 +1,9 @@
 from __future__ import annotations
 
-import sys
+import ast
+import inspect
 
+from skills_learning import store as skill_store_module
 from skills_learning.models import (
     LOCAL_SYNTHESIS_SOURCE,
     FlowStep,
@@ -11,6 +13,25 @@ from skills_learning.models import (
 from skills_learning.store import SkillStore
 
 
+def _imported_module_names(module) -> set[str]:
+    """Statically collect every module name imported by ``module``'s source.
+
+    Covers both ``import x.y`` and ``from x.y import z`` forms so the check
+    actually fails if a catalog-subscription import is ever added, instead
+    of only checking whether some module happened to already be imported by
+    something else in the running interpreter.
+    """
+    source = inspect.getsource(module)
+    tree = ast.parse(source)
+    names: set[str] = set()
+    for node in ast.walk(tree):
+        if isinstance(node, ast.Import):
+            names.update(alias.name for alias in node.names)
+        elif isinstance(node, ast.ImportFrom) and node.module:
+            names.add(node.module)
+    return names
+
+
 def _skill(
     *,
     name: str = "search web",
@@ -35,7 +56,19 @@ def test_skill_store_enforces_local_synthesis_source_without_catalog_write_path(
     stored = store.create_version(_skill())
 
     assert stored.source == LOCAL_SYNTHESIS_SOURCE
-    assert "skills.catalog" not in sys.modules
+
+    imported_modules = _imported_module_names(skill_store_module)
+    catalog_coupled = {
+        name
+        for name in imported_modules
+        if "catalog" in name.lower() or "sync_client" in name.lower()
+    }
+    assert catalog_coupled == set(), (
+        "skills_learning/store.py must not import from the "
+        "skill-catalog-subscription store/sync-client (D6: locally-authored "
+        "skills live in their own store, composed only in prose); found: "
+        f"{catalog_coupled}"
+    )
 
 
 def test_skill_store_keeps_version_chain_and_returns_latest_by_name() -> None: