mcp: remove the restart/kill/start/update/get_logs agent verbs

Container lifecycle from inside an agent goes away: an agent no longer
starts, stops, restarts or rebuilds a container in its subtree, and no
longer reads another container's journal. Those are operator actions —
the dashboard and hivectl keep their own paths to the same job-queue
and hive-priv plumbing, which is why none of that machinery is removed
here, only the five MCP verbs and what they alone reached.

What went with them: the `Request` variants and `Response::Logs` on the
agent socket, the five tool definitions and their arg structs, the four
lifecycle handlers plus `handle_get_logs`, and `require_descendant` —
the topology guard those five were the only remaining callers of.
`ToolGroup::Diagnostics` goes too: `get_logs` was its only tool, so it
would otherwise be a grantable group that grants nothing. `lifecycle`
stays, now carrying `list_containers` alone.

An agent that gets a `needs_update` or `container_crash` helper event
has no remedy of its own left, so the system prompt and the docs now
send it to the operator instead of to a tool that no longer exists.

Refs #4480

docs/tools/scheduling.md

@@ -61,17 +61,6 @@ and `last_result`, `next_fire_at_unix`, `interval_seconds`. Use to
 look up an id before cancelling, or to audit upcoming wake-ups in
 your subtree.
 
-## `diagnostics` tool group
-
-### `get_logs(agent, lines?)`
-
-Fetch recent journal lines for a sub-agent container. Useful for
-diagnosing MCP-registration failures, startup crashes, plugin install
-errors, or any harness issue you can't see from inside the container.
-
-Pass the plain logical agent name (for example `"gui"`) — hive-c0re resolves
-the machine name (`h-<name>`). `lines` defaults to 50, host-capped at 500.
-
 ## `read_host_journal` capability
 
 Capability-gated (not a tool group) — the operator enables it in the