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_cassandra_integration.py

@@ -115,7 +115,7 @@ class TestCassandraIntegration:
         
         # Create test message
         storage_message = Triples(
-            metadata=Metadata(user="testuser", collection="testcol"),
+            metadata=Metadata(collection="testcol"),
             triples=[
                 Triple(
                     s=Term(type=IRI, iri="http://example.org/person1"),
@@ -178,7 +178,7 @@ class TestCassandraIntegration:
         
         # Store test data for querying
         query_test_message = Triples(
-            metadata=Metadata(user="testuser", collection="testcol"),
+            metadata=Metadata(collection="testcol"),
             triples=[
                 Triple(
                     s=Term(type=IRI, iri="http://example.org/alice"),
@@ -212,7 +212,6 @@ class TestCassandraIntegration:
             p=None,  # None for wildcard
             o=None,  # None for wildcard
             limit=10,
-            user="testuser",
             collection="testcol"
         )
         s_results = await query_processor.query_triples(s_query)
@@ -232,7 +231,6 @@ class TestCassandraIntegration:
             p=Term(type=IRI, iri="http://example.org/knows"),
             o=None,  # None for wildcard
             limit=10,
-            user="testuser",
             collection="testcol"
         )
         p_results = await query_processor.query_triples(p_query)
@@ -259,7 +257,7 @@ class TestCassandraIntegration:
         # Create multiple coroutines for concurrent storage
         async def store_person_data(person_id, name, age, department):
             message = Triples(
-                metadata=Metadata(user="concurrent_test", collection="people"),
+                metadata=Metadata(collection="people"),
                 triples=[
                     Triple(
                         s=Term(type=IRI, iri=f"http://example.org/{person_id}"),
@@ -329,7 +327,7 @@ class TestCassandraIntegration:
         
         # Create a knowledge graph about a company
         company_graph = Triples(
-            metadata=Metadata(user="integration_test", collection="company"),
+            metadata=Metadata(collection="company"),
             triples=[
                 # People and their types
                 Triple(