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.

specs/api/paths/collection-management.yaml

@@ -10,11 +10,10 @@ post:
     Collections are organizational units for grouping:
     - Documents in the librarian
     - Knowledge cores
-    - User data
+    - Workspace data
 
     Each collection has:
-    - **user**: Owner identifier
-    - **collection**: Unique collection ID
+    - **collection**: Unique collection ID (within the workspace)
     - **name**: Human-readable display name
     - **description**: Purpose and contents
     - **tags**: Labels for filtering and organization
@@ -22,7 +21,7 @@ post:
     ## Operations
 
     ### list-collections
-    List all collections for a user. Optionally filter by tags and limit results.
+    List all collections in the workspace. Optionally filter by tags and limit results.
     Returns array of collection metadata.
 
     ### update-collection
@@ -30,7 +29,7 @@ post:
     If it exists, metadata is updated. Allows setting name, description, and tags.
 
     ### delete-collection
-    Delete a collection by user and collection ID. This removes the metadata but
+    Delete a collection by collection ID. This removes the metadata but
     typically does not delete the associated data (documents, knowledge cores).
 
   operationId: collectionManagementService
@@ -44,22 +43,19 @@ post:
           $ref: '../components/schemas/collection/CollectionRequest.yaml'
         examples:
           listCollections:
-            summary: List all collections for user
+            summary: List all collections in workspace
             value:
               operation: list-collections
-              user: alice
           listCollectionsFiltered:
             summary: List collections filtered by tags
             value:
               operation: list-collections
-              user: alice
               tag-filter: ["research", "AI"]
               limit: 50
           updateCollection:
             summary: Create/update collection
             value:
               operation: update-collection
-              user: alice
               collection: research
               name: Research Papers
               description: Academic research papers on AI and ML
@@ -69,7 +65,6 @@ post:
             summary: Delete collection
             value:
               operation: delete-collection
-              user: alice
               collection: research
   responses:
     '200':
@@ -84,13 +79,11 @@ post:
               value:
                 timestamp: "2024-01-15T10:30:00Z"
                 collections:
-                  - user: alice
-                    collection: research
+                  - collection: research
                     name: Research Papers
                     description: Academic research papers on AI and ML
                     tags: ["research", "AI", "academic"]
-                  - user: alice
-                    collection: personal
+                  - collection: personal
                     name: Personal Documents
                     description: Personal notes and documents
                     tags: ["personal"]