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/integration/test_cassandra_config_end_to_end.py

@@ -40,14 +40,13 @@ class TestEndToEndConfigurationFlow:
 
             # Create a mock message to trigger TrustGraph creation
             mock_message = MagicMock()
-            mock_message.metadata.user = 'test_user'
             mock_message.metadata.collection = 'test_collection'
             mock_message.triples = []
 
             # Mock collection_exists to return True
             with patch('trustgraph.direct.cassandra_kg.KnowledgeGraph.collection_exists', return_value=True):
                 # This should create TrustGraph with environment config
-                await processor.store_triples(mock_message)
+                await processor.store_triples('test_user', mock_message)
             
             # Verify Cluster was created with correct hosts
             mock_cluster.assert_called_once()
@@ -144,13 +143,12 @@ class TestConfigurationPriorityEndToEnd:
             
             # Trigger TrustGraph creation
             mock_message = MagicMock()
-            mock_message.metadata.user = 'test_user'
             mock_message.metadata.collection = 'test_collection'
             mock_message.triples = []
 
             # Mock collection_exists to return True
             with patch('trustgraph.direct.cassandra_kg.KnowledgeGraph.collection_exists', return_value=True):
-                await processor.store_triples(mock_message)
+                await processor.store_triples('test_user', mock_message)
             
             # Should use CLI parameters, not environment
             mock_cluster.assert_called_once()
@@ -201,7 +199,6 @@ class TestConfigurationPriorityEndToEnd:
             
             # Mock query to trigger TrustGraph creation
             mock_query = MagicMock()
-            mock_query.user = 'default_user'
             mock_query.collection = 'default_collection'
             mock_query.s = None
             mock_query.p = None
@@ -213,7 +210,7 @@ class TestConfigurationPriorityEndToEnd:
             mock_tg_instance.get_all.return_value = []
             processor.tg = mock_tg_instance
             
-            await processor.query_triples(mock_query)
+            await processor.query_triples('default_user', mock_query)
             
             # Should use defaults
             mock_cluster.assert_called_once()
@@ -244,13 +241,12 @@ class TestNoBackwardCompatibilityEndToEnd:
         
         # Trigger TrustGraph creation
         mock_message = MagicMock()
-        mock_message.metadata.user = 'legacy_user'
         mock_message.metadata.collection = 'legacy_collection'
         mock_message.triples = []
 
         # Mock collection_exists to return True
         with patch('trustgraph.direct.cassandra_kg.KnowledgeGraph.collection_exists', return_value=True):
-            await processor.store_triples(mock_message)
+            await processor.store_triples('legacy_user', mock_message)
         
         # Should use defaults since old parameters are not recognized
         mock_cluster.assert_called_once()
@@ -302,13 +298,12 @@ class TestNoBackwardCompatibilityEndToEnd:
         
         # Trigger TrustGraph creation
         mock_message = MagicMock()
-        mock_message.metadata.user = 'precedence_user'
         mock_message.metadata.collection = 'precedence_collection'
         mock_message.triples = []
 
         # Mock collection_exists to return True
         with patch('trustgraph.direct.cassandra_kg.KnowledgeGraph.collection_exists', return_value=True):
-            await processor.store_triples(mock_message)
+            await processor.store_triples('precedence_user', mock_message)
         
         # Should use new parameters, not old ones
         mock_cluster.assert_called_once()
@@ -354,13 +349,12 @@ class TestMultipleHostsHandling:
         
         # Trigger TrustGraph creation
         mock_message = MagicMock()
-        mock_message.metadata.user = 'single_user'
         mock_message.metadata.collection = 'single_collection'
         mock_message.triples = []
 
         # Mock collection_exists to return True
         with patch('trustgraph.direct.cassandra_kg.KnowledgeGraph.collection_exists', return_value=True):
-            await processor.store_triples(mock_message)
+            await processor.store_triples('single_user', mock_message)
         
         # Single host should be converted to list
         mock_cluster.assert_called_once()