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_document_rag_streaming_integration.py

@@ -107,7 +107,6 @@ class TestDocumentRagStreaming:
         # Act
         result = await document_rag_streaming.query(
             query=query,
-            user="test_user",
             collection="test_collection",
             doc_limit=10,
             streaming=True,
@@ -141,7 +140,6 @@ class TestDocumentRagStreaming:
         # Act - Non-streaming
         non_streaming_result = await document_rag_streaming.query(
             query=query,
-            user=user,
             collection=collection,
             doc_limit=doc_limit,
             streaming=False
@@ -155,7 +153,6 @@ class TestDocumentRagStreaming:
 
         streaming_result = await document_rag_streaming.query(
             query=query,
-            user=user,
             collection=collection,
             doc_limit=doc_limit,
             streaming=True,
@@ -178,7 +175,6 @@ class TestDocumentRagStreaming:
         # Act
         result = await document_rag_streaming.query(
             query="test query",
-            user="test_user",
             collection="test_collection",
             doc_limit=5,
             streaming=True,
@@ -200,7 +196,6 @@ class TestDocumentRagStreaming:
         # Arrange & Act
         result = await document_rag_streaming.query(
             query="test query",
-            user="test_user",
             collection="test_collection",
             doc_limit=5,
             streaming=True,
@@ -223,7 +218,6 @@ class TestDocumentRagStreaming:
         # Act
         result = await document_rag_streaming.query(
             query="unknown topic",
-            user="test_user",
             collection="test_collection",
             doc_limit=10,
             streaming=True,
@@ -247,7 +241,6 @@ class TestDocumentRagStreaming:
         with pytest.raises(Exception) as exc_info:
             await document_rag_streaming.query(
                 query="test query",
-                user="test_user",
                 collection="test_collection",
                 doc_limit=5,
                 streaming=True,
@@ -272,7 +265,6 @@ class TestDocumentRagStreaming:
             # Act
             result = await document_rag_streaming.query(
                 query="test query",
-                user="test_user",
                 collection="test_collection",
                 doc_limit=limit,
                 streaming=True,
@@ -300,7 +292,6 @@ class TestDocumentRagStreaming:
         # Act
         await document_rag_streaming.query(
             query="test query",
-            user=user,
             collection=collection,
             doc_limit=10,
             streaming=True,
@@ -309,5 +300,4 @@ class TestDocumentRagStreaming:
 
         # Assert - Verify user/collection were passed to document embeddings client
         call_args = mock_doc_embeddings_client.query.call_args
-        assert call_args.kwargs['user'] == user
         assert call_args.kwargs['collection'] == collection