remove the list_containers and request_update_meta_inputs MCP tools

Both agent-facing tools go away end to end, with no replacement. This is
an intentional capability removal: agents can no longer enumerate their
own subtree, and can no longer queue a meta-flake input bump.

The system prompt and docs/tools/lifecycle.md land in this same commit
on purpose. A tool named in the prompt but absent from the server makes
agents confidently call something that doesn't exist, and the failure
then surfaces far from its cause.

Removed:

- MCP registrations and bodies (hive-agent-mcp), plus the now-unused
  UpdateMetaInputsArgs.
- Wire variants Request::ListDescendants,
  Request::RequestUpdateMetaInputs and Response::Containers, plus
  ContainerInfo, whose only consumer was that response.
- hive-c0re's handle_list_descendants (its whole module) and
  handle_request_update_meta_inputs, the two dispatch arms, and the
  require_group(agent, "approvals", ...) gate on the meta-inputs verb.
- The stream_enrich emoji entry and argument formatter.
- docs/tools/lifecycle.md (both tools it documented are gone), its two
  referrers, the tool-group tables and the agent-hierarchy prose.

Tool groups are kept, deliberately. ToolGroup::Lifecycle listed exactly
one tool and now lists none — it is vestigial, but the variant stays so
existing meta/capabilities.json grants still parse; retiring it is a
separate decision. ToolGroup::Approvals also listed exactly one tool,
but the group is NOT dead: check_can_cancel_approval still gates
cancel_loose_end's approval-cancel arm on it server-side.

ApprovalKind::UpdateMetaInputs stays too. Nothing in production code
produces it any more, but pre-existing approval rows may still carry it,
and the operator's own path to a meta update is unaffected — the
dashboard's POST /api/meta-update inserts the meta_update job directly,
bypassing approvals entirely.

The two format_ack tests in hive-agent-mcp that named
request_update_meta_inputs were only using it as a label string while
exercising the generic OkWarn/Ok renderer, so they are retargeted to a
surviving tool rather than deleted.

Note hive-c0re's priv_client::list_containers is a different thing (the
host-side privileged container listing behind hive-priv) and is
untouched.

Closes #4591

hive-agent-mcp/src/mcp/render.rs

@@ -643,12 +643,12 @@ mod tests {
             Ok(hive_core_agent_sock::Response::OkWarn {
                 warnings: vec!["name is reserved".to_owned(), "second thing".to_owned()],
             }),
-            "request_update_meta_inputs",
-            "update_meta_inputs approval queued".to_owned(),
+            "request_schedule_prompt",
+            "schedule approval queued".to_owned(),
         );
         // The operation HAPPENED — dropping the success line would read as a
         // failure and invite a retry that queues a second approval.
-        assert!(out.starts_with("update_meta_inputs approval queued"));
+        assert!(out.starts_with("schedule approval queued"));
         assert!(out.contains("⚠️ name is reserved"));
         assert!(out.contains("⚠️ second thing"));
     }
@@ -659,7 +659,7 @@ mod tests {
         // warning marker would pass the test above.
         let out = format_ack(
             Ok(hive_core_agent_sock::Response::Ok),
-            "request_update_meta_inputs",
+            "request_schedule_prompt",
             "queued".to_owned(),
         );
         assert_eq!(out, "queued");