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/knowledge.yaml

@@ -12,12 +12,12 @@ post:
     - **Graph Embeddings**: Vector embeddings for entities
     - **Metadata**: Descriptive information about the knowledge
 
-    Each core has an ID, user, and collection for organization.
+    Each core has an ID and collection for organization (within the workspace).
 
     ## Operations
 
     ### list-kg-cores
-    List all knowledge cores for a user. Returns array of core IDs.
+    List all knowledge cores in the workspace. Returns array of core IDs.
 
     ### get-kg-core
     Retrieve a knowledge core by ID. Returns triples and/or graph embeddings.
@@ -58,7 +58,6 @@ post:
             summary: List knowledge cores
             value:
               operation: list-kg-cores
-              user: alice
           getKnowledgeCore:
             summary: Get knowledge core
             value:
@@ -71,7 +70,6 @@ post:
               triples:
                 metadata:
                   id: core-123
-                  user: alice
                   collection: default
                   metadata:
                     - s: {v: "https://example.com/core-123", e: true}
@@ -91,7 +89,6 @@ post:
               graph-embeddings:
                 metadata:
                   id: core-123
-                  user: alice
                   collection: default
                   metadata: []
                 entities:
@@ -106,7 +103,6 @@ post:
               triples:
                 metadata:
                   id: core-456
-                  user: bob
                   collection: research
                   metadata: []
                 triples:
@@ -116,7 +112,6 @@ post:
               graph-embeddings:
                 metadata:
                   id: core-456
-                  user: bob
                   collection: research
                   metadata: []
                 entities:
@@ -127,7 +122,6 @@ post:
             value:
               operation: delete-kg-core
               id: core-123
-              user: alice
           loadKnowledgeCore:
             summary: Load core into flow
             value:
@@ -161,7 +155,6 @@ post:
                 triples:
                   metadata:
                     id: core-123
-                    user: alice
                     collection: default
                     metadata:
                       - s: {v: "https://example.com/core-123", e: true}
@@ -177,7 +170,6 @@ post:
                 graph-embeddings:
                   metadata:
                     id: core-123
-                    user: alice
                     collection: default
                     metadata: []
                   entities: