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_graph_rag_streaming_integration.py

@@ -116,7 +116,6 @@ class TestGraphRagStreaming:
         # Act - query() returns response, provenance via callback
         response = await graph_rag_streaming.query(
             query=query,
-            user="test_user",
             collection="test_collection",
             streaming=True,
             chunk_callback=collector.collect,
@@ -154,7 +153,6 @@ class TestGraphRagStreaming:
         # Act - Non-streaming
         non_streaming_response = await graph_rag_streaming.query(
             query=query,
-            user=user,
             collection=collection,
             streaming=False
         )
@@ -167,7 +165,6 @@ class TestGraphRagStreaming:
 
         streaming_response = await graph_rag_streaming.query(
             query=query,
-            user=user,
             collection=collection,
             streaming=True,
             chunk_callback=collect
@@ -189,7 +186,6 @@ class TestGraphRagStreaming:
         # Act
         response = await graph_rag_streaming.query(
             query="test query",
-            user="test_user",
             collection="test_collection",
             streaming=True,
             chunk_callback=callback
@@ -209,7 +205,6 @@ class TestGraphRagStreaming:
         # Arrange & Act
         response = await graph_rag_streaming.query(
             query="test query",
-            user="test_user",
             collection="test_collection",
             streaming=True,
             chunk_callback=None  # No callback provided
@@ -231,7 +226,6 @@ class TestGraphRagStreaming:
         # Act
         response = await graph_rag_streaming.query(
             query="unknown topic",
-            user="test_user",
             collection="test_collection",
             streaming=True,
             chunk_callback=callback
@@ -253,7 +247,6 @@ class TestGraphRagStreaming:
         with pytest.raises(Exception) as exc_info:
             await graph_rag_streaming.query(
                 query="test query",
-                user="test_user",
                 collection="test_collection",
                 streaming=True,
                 chunk_callback=callback
@@ -273,7 +266,6 @@ class TestGraphRagStreaming:
         # Act
         await graph_rag_streaming.query(
             query="test query",
-            user="test_user",
             collection="test_collection",
             entity_limit=entity_limit,
             triple_limit=triple_limit,