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.py

@@ -92,14 +92,13 @@ class TestQuery:
         # Initialize Query with defaults
         query = Query(
             rag=mock_rag,
-            user="test_user",
+            workspace="test_workspace",
             collection="test_collection",
             verbose=False
         )
 
         # Verify initialization
         assert query.rag == mock_rag
-        assert query.user == "test_user"
         assert query.collection == "test_collection"
         assert query.verbose is False
         assert query.doc_limit == 20  # Default value
@@ -112,7 +111,7 @@ class TestQuery:
         # Initialize Query with custom doc_limit
         query = Query(
             rag=mock_rag,
-            user="custom_user",
+            workspace="test_workspace",
             collection="custom_collection",
             verbose=True,
             doc_limit=50
@@ -120,7 +119,6 @@ class TestQuery:
 
         # Verify initialization
         assert query.rag == mock_rag
-        assert query.user == "custom_user"
         assert query.collection == "custom_collection"
         assert query.verbose is True
         assert query.doc_limit == 50
@@ -137,7 +135,7 @@ class TestQuery:
 
         query = Query(
             rag=mock_rag,
-            user="test_user",
+            workspace="test_workspace",
             collection="test_collection",
             verbose=False
         )
@@ -162,7 +160,7 @@ class TestQuery:
 
         query = Query(
             rag=mock_rag,
-            user="test_user",
+            workspace="test_workspace",
             collection="test_collection",
             verbose=False
         )
@@ -184,7 +182,7 @@ class TestQuery:
 
         query = Query(
             rag=mock_rag,
-            user="test_user",
+            workspace="test_workspace",
             collection="test_collection",
             verbose=False
         )
@@ -223,7 +221,7 @@ class TestQuery:
 
         query = Query(
             rag=mock_rag,
-            user="test_user",
+            workspace="test_workspace",
             collection="test_collection",
             verbose=False,
             doc_limit=15
@@ -240,7 +238,6 @@ class TestQuery:
         mock_doc_embeddings_client.query.assert_called_once_with(
             vector=[0.1, 0.2, 0.3],
             limit=15,
-            user="test_user",
             collection="test_collection"
         )
 
@@ -286,7 +283,6 @@ class TestQuery:
 
         result = await document_rag.query(
             query="test query",
-            user="test_user",
             collection="test_collection",
             doc_limit=10
         )
@@ -304,7 +300,6 @@ class TestQuery:
         mock_doc_embeddings_client.query.assert_called_once_with(
             vector=[0.1, 0.2, 0.3],
             limit=10,
-            user="test_user",
             collection="test_collection"
         )
 
@@ -350,7 +345,6 @@ class TestQuery:
         mock_doc_embeddings_client.query.assert_called_once_with(
             vector=[[0.1, 0.2]],
             limit=20,  # Default doc_limit
-            user="trustgraph",  # Default user
             collection="default"  # Default collection
         )
 
@@ -380,7 +374,7 @@ class TestQuery:
 
         query = Query(
             rag=mock_rag,
-            user="test_user",
+            workspace="test_workspace",
             collection="test_collection",
             verbose=True,
             doc_limit=5
@@ -453,7 +447,7 @@ class TestQuery:
 
         query = Query(
             rag=mock_rag,
-            user="test_user",
+            workspace="test_workspace",
             collection="test_collection",
             verbose=False
         )
@@ -509,7 +503,7 @@ class TestQuery:
 
         query = Query(
             rag=mock_rag,
-            user="test_user",
+            workspace="test_workspace",
             collection="test_collection",
             verbose=True
         )
@@ -558,7 +552,6 @@ class TestQuery:
 
         result = await document_rag.query(
             query=query_text,
-            user="research_user",
             collection="ml_knowledge",
             doc_limit=25
         )
@@ -619,7 +612,7 @@ class TestQuery:
 
         query = Query(
             rag=mock_rag,
-            user="test_user",
+            workspace="test_workspace",
             collection="test_collection",
             verbose=False,
             doc_limit=10