feat: workspace-based multi-tenancy, replacing user as tenancy axis (#840)

Introduces `workspace` as the isolation boundary for config, flows,
library, and knowledge data. Removes `user` as a schema-level field
throughout the code, API specs, and tests; workspace provides the
same separation more cleanly at the trusted flow.workspace layer
rather than through client-supplied message fields.

Design
------
- IAM tech spec (docs/tech-specs/iam.md) documents current state,
  proposed auth/access model, and migration direction.
- Data ownership model (docs/tech-specs/data-ownership-model.md)
  captures the workspace/collection/flow hierarchy.

Schema + messaging
------------------
- Drop `user` field from AgentRequest/Step, GraphRagQuery,
  DocumentRagQuery, Triples/Graph/Document/Row EmbeddingsRequest,
  Sparql/Rows/Structured QueryRequest, ToolServiceRequest.
- Keep collection/workspace routing via flow.workspace at the
  service layer.
- Translators updated to not serialise/deserialise user.

API specs
---------
- OpenAPI schemas and path examples cleaned of user fields.
- Websocket async-api messages updated.
- Removed the unused parameters/User.yaml.

Services + base
---------------
- Librarian, collection manager, knowledge, config: all operations
  scoped by workspace. Config client API takes workspace as first
  positional arg.
- `flow.workspace` set at flow start time by the infrastructure;
  no longer pass-through from clients.
- Tool service drops user-personalisation passthrough.

CLI + SDK
---------
- tg-init-workspace and workspace-aware import/export.
- All tg-* commands drop user args; accept --workspace.
- Python API/SDK (flow, socket_client, async_*, explainability,
  library) drop user kwargs from every method signature.

MCP server
----------
- All tool endpoints drop user parameters; socket_manager no longer
  keyed per user.

Flow service
------------
- Closure-based topic cleanup on flow stop: only delete topics
  whose blueprint template was parameterised AND no remaining
  live flow (across all workspaces) still resolves to that topic.
  Three scopes fall out naturally from template analysis:
    * {id} -> per-flow, deleted on stop
    * {blueprint} -> per-blueprint, kept while any flow of the
      same blueprint exists
    * {workspace} -> per-workspace, kept while any flow in the
      workspace exists
    * literal -> global, never deleted (e.g. tg.request.librarian)
  Fixes a bug where stopping a flow silently destroyed the global
  librarian exchange, wedging all library operations until manual
  restart.

RabbitMQ backend
----------------
- heartbeat=60, blocked_connection_timeout=300. Catches silently
  dead connections (broker restart, orphaned channels, network
  partitions) within ~2 heartbeat windows, so the consumer
  reconnects and re-binds its queue rather than sitting forever
  on a zombie connection.

Tests
-----
- Full test refresh: unit, integration, contract, provenance.
- Dropped user-field assertions and constructor kwargs across
  ~100 test files.
- Renamed user-collection isolation tests to workspace-collection.

tests/unit/test_retrieval/test_document_rag_service.py

@@ -1,6 +1,6 @@
 """
 Unit test for DocumentRAG service parameter passing fix.
-Tests that user and collection parameters from the message are correctly
+Tests that the collection parameter from the message is correctly
 passed to the DocumentRag.query() method.
 """
 
@@ -16,13 +16,13 @@ class TestDocumentRagService:
 
     @patch('trustgraph.retrieval.document_rag.rag.DocumentRag')
     @pytest.mark.asyncio
-    async def test_user_and_collection_parameters_passed_to_query(self, mock_document_rag_class):
+    async def test_collection_parameter_passed_to_query(self, mock_document_rag_class):
         """
-        Test that user and collection from message are passed to DocumentRag.query().
-        
-        This is a regression test for the bug where user/collection parameters
-        were ignored, causing wrong collection names like 'd_trustgraph_default_384'
-        instead of 'd_my_user_test_coll_1_384'.
+        Test that collection from message is passed to DocumentRag.query().
+
+        This is a regression test for the bug where the collection parameter
+        was ignored, causing wrong collection names like 'd_trustgraph_default_384'
+        instead of one that reflects the requested collection.
         """
         # Setup processor
         processor = Processor(
@@ -30,17 +30,16 @@ class TestDocumentRagService:
             id="test-processor",
             doc_limit=10
         )
-        
+
         # Setup mock DocumentRag instance
         mock_rag_instance = AsyncMock()
         mock_document_rag_class.return_value = mock_rag_instance
         mock_rag_instance.query.return_value = ("test response", {"in_token": None, "out_token": None, "model": None})
-        
-        # Setup message with custom user/collection
+
+        # Setup message with custom collection
         msg = MagicMock()
         msg.value.return_value = DocumentRagQuery(
             query="test query",
-            user="my_user",        # Custom user (not default "trustgraph")  
             collection="test_coll_1",  # Custom collection (not default "default")
             doc_limit=5
         )
@@ -64,7 +63,7 @@ class TestDocumentRagService:
         # Verify: DocumentRag.query was called with correct parameters
         mock_rag_instance.query.assert_called_once_with(
             "test query",
-            user="my_user",           # Must be from message, not hardcoded default
+            workspace=ANY,            # Workspace comes from flow.workspace (mock)
             collection="test_coll_1", # Must be from message, not hardcoded default
             doc_limit=5,
             explain_callback=ANY,     # Explainability callback is always passed
@@ -103,7 +102,6 @@ class TestDocumentRagService:
         msg = MagicMock()
         msg.value.return_value = DocumentRagQuery(
             query="What is a cat?",
-            user="trustgraph",
             collection="default",
             doc_limit=10,
             streaming=False  # Non-streaming mode