remove the get_host_journal MCP tool and its capability

swarm-logs covers every host-tier unit this tool could reach, so the
second, capability-gated path into host journald earns nothing and is
removed outright rather than disabled behind a flag.

Removed end to end: the MCP tool definition + handler, the
GetHostJournal/HostJournal wire variants, hive-c0re's
dispatch_host_journal handler, the ReadHostJournal capability, and the
harness-side capability->--allowedTools gate. get_host_journal was the
only capability that mapped to an MCP tool, so allowed_capability_tools
could only ever return an empty vec; it goes too rather than linger as a
function that provably does nothing.

capabilities::has_cap/caps_for stay: #4624 gave ManageRootAgent's
bind-mount enforcement (hive-c0re/src/lifecycle/host_config.rs) a
second caller of has_cap, so they're no longer callerless once this
lands on top of it.

hive-sh4re's journal module (JournalPriority) had no consumer outside
this tool and is deleted.

An existing capabilities.json still naming read_host_journal does not
error: capabilities::prune_unknown drops unrecognised names with a
warn!, and an agent left with no capabilities has its entry removed. No
migration step is needed.

Untouched: hive-c0re/src/dashboard/journal.rs's
read_host_journal_response, which matches the name but is the private
helper behind the operator-only GET /api/journal-host dashboard route
and carries no capability check.

docs/agent-lifecycle/persistence.md

@@ -399,7 +399,7 @@ Contents:
   `tool_groups::set_groups`; injected as `HIVE_TOOL_GROUPS` env
   var into each agent's container.
 - `capabilities.json` — per-agent capability grants
-  (`{ "atlas": ["read_host_journal"] }`). Written by
+  (`{ "ruth": ["manage_root_agent"] }`). Written by
   `capabilities::set_caps`; injected as `HIVE_CAPABILITIES` env
   var. Absent agents have no extra capabilities.
 - `resource-limits.json` — per-agent container resource overrides