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/contract/test_structured_data_contracts.py

@@ -93,7 +93,6 @@ class TestStructuredDataSchemaContracts:
         # Arrange
         metadata = Metadata(
             id="structured-data-001",
-            user="test_user",
             collection="test_collection",
         )
 
@@ -118,7 +117,6 @@ class TestStructuredDataSchemaContracts:
         # Arrange
         metadata = Metadata(
             id="extracted-obj-001",
-            user="test_user",
             collection="test_collection",
         )
 
@@ -143,7 +141,6 @@ class TestStructuredDataSchemaContracts:
         # Arrange
         metadata = Metadata(
             id="extracted-batch-001",
-            user="test_user",
             collection="test_collection",
         )
 
@@ -177,7 +174,6 @@ class TestStructuredDataSchemaContracts:
         # Arrange
         metadata = Metadata(
             id="extracted-empty-001",
-            user="test_user",
             collection="test_collection",
         )
 
@@ -277,7 +273,6 @@ class TestStructuredEmbeddingsContracts:
         # Arrange
         metadata = Metadata(
             id="struct-embed-001",
-            user="test_user",
             collection="test_collection",
         )
 
@@ -308,7 +303,7 @@ class TestStructuredDataSerializationContracts:
     def test_structured_data_submission_serialization(self):
         """Test StructuredDataSubmission serialization contract"""
         # Arrange
-        metadata = Metadata(id="test", user="user", collection="col")
+        metadata = Metadata(id="test", collection="col")
         submission_data = {
             "metadata": metadata,
             "format": "json",
@@ -323,7 +318,7 @@ class TestStructuredDataSerializationContracts:
     def test_extracted_object_serialization(self):
         """Test ExtractedObject serialization contract"""
         # Arrange
-        metadata = Metadata(id="test", user="user", collection="col")
+        metadata = Metadata(id="test", collection="col")
         object_data = {
             "metadata": metadata,
             "schema_name": "test_schema",
@@ -373,7 +368,7 @@ class TestStructuredDataSerializationContracts:
     def test_extracted_object_batch_serialization(self):
         """Test ExtractedObject batch serialization contract"""
         # Arrange
-        metadata = Metadata(id="test", user="user", collection="col")
+        metadata = Metadata(id="test", collection="col")
         batch_object_data = {
             "metadata": metadata,
             "schema_name": "test_schema",
@@ -392,7 +387,7 @@ class TestStructuredDataSerializationContracts:
     def test_extracted_object_empty_batch_serialization(self):
         """Test ExtractedObject empty batch serialization contract"""
         # Arrange
-        metadata = Metadata(id="test", user="user", collection="col")
+        metadata = Metadata(id="test", collection="col")
         empty_batch_data = {
             "metadata": metadata,
             "schema_name": "test_schema",