Phase 5.5-5.9: Plugin-Marketplace, Agent Memory, GraphRAG, Subagents, External Agent API

5.5 Plugin-Marketplace:
- New plugin: marketplace/ (models, routes, services, schemas, config)
- MarketplaceListing model (global, no tenant_id)
- Ed25519 signature verification via PluginSignature
- Endpoints: list, detail, install, verify, categories
- Config: MARKETPLACE_SERVER_URL setting

5.6 Agent Memory (persistent):
- New plugin: agent_memory/ (models, routes, services, schemas)
- AgentMemory model with embedding vector(768) + HNSW index
- store_memory() with auto-embedding
- retrieve_relevant_memories() with pgvector cosine similarity
- Semantic search endpoint

5.7 GraphRAG:
- New plugin: graph_rag/ (models, routes, services, provider, schemas)
- EntityRelationship model (source/target type+id, relationship_type, metadata)
- BFS graph traversal (bidirectional, configurable depth)
- GraphRAGSearchProvider registered in unified_search

5.8 Subagents / Multi-Agent:
- AgentCoordinator class (create_subtask, wait_for_subtask, aggregate, cancel)
- AgentSubtask model + migration 0002_agent_subtasks.sql
- 6 new API endpoints for subtask management
- Tools registered in AI tool registry

5.9 External Agent API:
- external_api.py: POST /run, GET /status, POST /stream (SSE)
- Bearer API token authentication
- Rate limiting: 10 req/min per token
- ExternalAgentRequest/Response schemas

3 new plugins registered in main.py and __init__.py
All files py_compile clean

app/plugins/builtins/automation/models.py

@@ -251,3 +251,40 @@ class AutomationRun(Base, TenantMixin):
         JSONB, nullable=False, default=dict
     )
     dry_run: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+
+
+class AgentSubtask(Base, TenantMixin):
+    """A subtask delegated from one agent to another for multi-agent orchestration."""
+
+    __tablename__ = "agent_subtasks"
+    __table_args__ = (
+        Index("ix_agent_subtasks_parent", "tenant_id", "parent_agent_id"),
+        Index("ix_agent_subtasks_child", "tenant_id", "child_agent_id"),
+        Index("ix_agent_subtasks_status", "tenant_id", "status"),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    parent_agent_id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True),
+        ForeignKey("automation_agent_definitions.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    child_agent_id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True),
+        ForeignKey("automation_agent_definitions.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    task_description: Mapped[str] = mapped_column(Text, nullable=False)
+    status: Mapped[str] = mapped_column(
+        String(20), nullable=False, default="pending"
+    )
+    result: Mapped[dict[str, Any]] = mapped_column(
+        JSONB, nullable=False, default=dict
+    )
+    completed_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )