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_versioning.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from skills_learning.config import SkillAuthoringConfig
 from skills_learning.models import FlowStep, FlowTemplateSkill, SkillMetadata
 from skills_learning.store import SkillStore
 from skills_learning.versioning import diff_flow_versions, store_synthesized_skill
@@ -70,3 +71,62 @@ def test_structural_divergence_creates_new_version_and_preserves_parent() -> Non
     assert result.skill.parent_version_id == first.id
     assert store.get_by_id(first.id) == first
     assert store.get_latest_by_name("search") == result.skill
+
+
+def _two_step_skill(*, text: str) -> FlowTemplateSkill:
+    return _skill(
+        steps=[
+            FlowStep("tap", {"x": 1}),
+            FlowStep("input_text", {"text": text}),
+        ]
+    )
+
+
+def test_argument_divergence_within_configured_tolerance_does_not_bump_version() -> None:
+    store = SkillStore()
+    first = store.create_version(_two_step_skill(text="coffee"))
+    candidate = _two_step_skill(text="tea")
+    # Half (1 of 2) step positions diverge; tolerance allows up to half.
+    config = SkillAuthoringConfig(divergence_tolerance=0.5)
+
+    diff = diff_flow_versions(first.steps, candidate.steps, config=config)
+    assert diff.structural_divergence is False
+    assert diff.argument_divergence_fraction == 0.5
+
+    result = store_synthesized_skill(store, candidate, config=config)
+
+    assert result.created_new_version is False
+    assert result.skill.version == first.version
+    assert result.skill.id == first.id
+
+
+def test_argument_divergence_beyond_configured_tolerance_bumps_version() -> None:
+    store = SkillStore()
+    first = store.create_version(_two_step_skill(text="coffee"))
+    candidate = _two_step_skill(text="tea")
+    # Half (1 of 2) step positions diverge; tolerance only allows less than that.
+    config = SkillAuthoringConfig(divergence_tolerance=0.3)
+
+    diff = diff_flow_versions(first.steps, candidate.steps, config=config)
+    assert diff.structural_divergence is True
+    assert diff.argument_divergence_fraction == 0.5
+
+    result = store_synthesized_skill(store, candidate, config=config)
+
+    assert result.created_new_version is True
+    assert result.skill.version == 2
+    assert result.skill.parent_version_id == first.id
+
+
+def test_argument_divergence_without_config_never_bumps_version() -> None:
+    # Backward-compatible default: no config supplied means unlimited
+    # tolerance, matching pre-existing behavior of absorbing any argument
+    # divergence as a parameter update rather than a new version.
+    store = SkillStore()
+    first = store.create_version(_two_step_skill(text="coffee"))
+    candidate = _two_step_skill(text="tea")
+
+    result = store_synthesized_skill(store, candidate)
+
+    assert result.created_new_version is False
+    assert result.skill.version == first.version