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_tables/test_knowledge_table_store.py

@@ -45,12 +45,9 @@ class TestGetGraphEmbeddings:
         with `vector=` (singular) — the schema field name. A previous
         version used `vectors=` and TypeError'd at runtime.
         """
-        # Arrange — fake row matching the get_triples_stmt result shape:
-        #   row[0..2] are unused by the method, row[3] is the entities blob
         fake_row = (
             None, None, None,
             [
-                # ((value, is_uri), vector)
                 (("http://example.org/alice", True), [0.1, 0.2, 0.3]),
                 (("http://example.org/bob", True), [0.4, 0.5, 0.6]),
                 (("a literal entity", False), [0.7, 0.8, 0.9]),
@@ -67,14 +64,8 @@ class TestGetGraphEmbeddings:
         async def receiver(msg):
             received.append(msg)
 
-        # Act
-        await store.get_graph_embeddings(
-            user="alice",
-            document_id="doc-1",
-            receiver=receiver,
-        )
+        await store.get_graph_embeddings("alice", "doc-1", receiver)
 
-        # Assert
         mock_async_execute.assert_called_once_with(
             store.cassandra,
             store.get_graph_embeddings_stmt,
@@ -86,7 +77,6 @@ class TestGetGraphEmbeddings:
         assert isinstance(ge, GraphEmbeddings)
         assert isinstance(ge.metadata, Metadata)
         assert ge.metadata.id == "doc-1"
-        assert ge.metadata.user == "alice"
 
         assert len(ge.entities) == 3
         assert all(isinstance(e, EntityEmbeddings) for e in ge.entities)
@@ -122,7 +112,7 @@ class TestGetGraphEmbeddings:
         async def receiver(msg):
             received.append(msg)
 
-        await store.get_graph_embeddings("u", "d", receiver)
+        await store.get_graph_embeddings("w", "d", receiver)
 
         assert len(received) == 1
         assert received[0].entities == []
@@ -149,7 +139,7 @@ class TestGetGraphEmbeddings:
         async def receiver(msg):
             received.append(msg)
 
-        await store.get_graph_embeddings("u", "d", receiver)
+        await store.get_graph_embeddings("w", "d", receiver)
 
         assert len(received) == 2
         assert received[0].entities[0].entity.iri == "http://example.org/a"
@@ -194,7 +184,6 @@ class TestGetTriples:
         assert isinstance(triples_msg, Triples)
         assert isinstance(triples_msg.metadata, Metadata)
         assert triples_msg.metadata.id == "doc-1"
-        assert triples_msg.metadata.user == "alice"
 
         assert len(triples_msg.triples) == 1
         t = triples_msg.triples[0]