feat(skill-catalog-subscription): synced catalog + HTTP sync + MCP tools

Consumes the external Subscription Platform as source of truth for skill
content; reuses skills_learning domain models (extended with KnowledgeSkill)
and workflow.skill_exec resolver. HTTP/MCP deps land in api/ per
CONSTITUTION.md; synced skills use a physically separate SQLite file
(tasks/skills.sqlite3) to preserve the skill-authoring capability boundary.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

skills_learning/__init__.py

@@ -1,12 +1,19 @@
 """Local skill learning from completed task timelines."""
 
 from skills_learning.config import SkillAuthoringConfig, load_config
-from skills_learning.models import FlowStep, FlowTemplateSkill, Skill, SkillMetadata
+from skills_learning.models import (
+    FlowStep,
+    FlowTemplateSkill,
+    KnowledgeSkill,
+    Skill,
+    SkillMetadata,
+)
 from skills_learning.store import SkillStore, get_default_store
 
 __all__ = [
     "FlowStep",
     "FlowTemplateSkill",
+    "KnowledgeSkill",
     "Skill",
     "SkillAuthoringConfig",
     "SkillMetadata",