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_reliability/test_metadata_preservation.py

@@ -30,7 +30,7 @@ class TestDocumentMetadataTranslator:
             "title": "Test Document",
             "comments": "No comments",
             "metadata": [],
-            "user": "alice",
+            "workspace": "alice",
             "tags": ["finance", "q4"],
             "parent-id": "doc-100",
             "document-type": "page",
@@ -40,14 +40,14 @@ class TestDocumentMetadataTranslator:
         assert obj.time == 1710000000
         assert obj.kind == "application/pdf"
         assert obj.title == "Test Document"
-        assert obj.user == "alice"
+        assert obj.workspace == "alice"
         assert obj.tags == ["finance", "q4"]
         assert obj.parent_id == "doc-100"
         assert obj.document_type == "page"
 
         wire = self.tx.encode(obj)
         assert wire["id"] == "doc-123"
-        assert wire["user"] == "alice"
+        assert wire["workspace"] == "alice"
         assert wire["parent-id"] == "doc-100"
         assert wire["document-type"] == "page"
 
@@ -80,10 +80,10 @@ class TestDocumentMetadataTranslator:
 
     def test_falsy_fields_omitted_from_wire(self):
         """Empty string fields should be omitted from wire format."""
-        obj = DocumentMetadata(id="", time=0, user="")
+        obj = DocumentMetadata(id="", time=0, workspace="")
         wire = self.tx.encode(obj)
         assert "id" not in wire
-        assert "user" not in wire
+        assert "workspace" not in wire
 
 
 # ---------------------------------------------------------------------------
@@ -101,7 +101,7 @@ class TestProcessingMetadataTranslator:
             "document-id": "doc-123",
             "time": 1710000000,
             "flow": "default",
-            "user": "alice",
+            "workspace": "alice",
             "collection": "my-collection",
             "tags": ["tag1"],
         }
@@ -109,20 +109,20 @@ class TestProcessingMetadataTranslator:
         assert obj.id == "proc-1"
         assert obj.document_id == "doc-123"
         assert obj.flow == "default"
-        assert obj.user == "alice"
+        assert obj.workspace == "alice"
         assert obj.collection == "my-collection"
         assert obj.tags == ["tag1"]
 
         wire = self.tx.encode(obj)
         assert wire["id"] == "proc-1"
         assert wire["document-id"] == "doc-123"
-        assert wire["user"] == "alice"
+        assert wire["workspace"] == "alice"
         assert wire["collection"] == "my-collection"
 
     def test_missing_fields_use_defaults(self):
         obj = self.tx.decode({})
         assert obj.id is None
-        assert obj.user is None
+        assert obj.workspace is None
         assert obj.collection is None
 
     def test_tags_none_omitted(self):
@@ -135,10 +135,10 @@ class TestProcessingMetadataTranslator:
         wire = self.tx.encode(obj)
         assert wire["tags"] == []
 
-    def test_user_and_collection_preserved(self):
+    def test_workspace_and_collection_preserved(self):
         """Core pipeline routing fields must survive round-trip."""
-        data = {"user": "bob", "collection": "research"}
+        data = {"workspace": "bob", "collection": "research"}
         obj = self.tx.decode(data)
         wire = self.tx.encode(obj)
-        assert wire["user"] == "bob"
+        assert wire["workspace"] == "bob"
         assert wire["collection"] == "research"