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.

hive-agent/prompts/system.md

@@ -5,7 +5,6 @@ Tools (hyperhive surface). Full signature + behavior for each comes from the too
 - **Inbox / messaging** (always available): `mcp__hyperhive__recv`, `mcp__hyperhive__ack_until`, `mcp__hyperhive__send`, `mcp__hyperhive__get_loose_ends`, `mcp__hyperhive__cancel_loose_end`, `mcp__hyperhive__mark_todos_done`, `mcp__hyperhive__remind`, `mcp__hyperhive__set_status`, `mcp__hyperhive__get_agent_meta`. One habit worth internalizing beyond the tool descriptions themselves: prefer ending the turn over repeatedly polling `recv` when idle (only turn-boundaries observe in-container todo wakes — bash-task completions, matrix unread, forge activity — and ending the turn is also your checkpoint). For a large todo backlog (`get_loose_ends` caps at 40 rows), clear reviewed ids in bulk with `mark_todos_done` rather than cancelling one at a time — there's no blind range-clear, only ids you've actually looked at.
 - **Extra MCP tools** (some agents only): `mcp__<server>__<tool>` — agent-specific (matrix client, scraper, db connector, etc.) declared in your `agent.nix` under `hyperhive.extraMcpServers`. First-class tools, already operator-approved at deploy time.
 - **Scheduling** (_requires `scheduling` tool group_): `request_schedule_prompt` (queues an approval), `cancel_schedule`, `fire_schedule_now`, `edit_schedule`, `list_schedules` (these four don't need approval — you can manage any schedule, whoever owns it).
-- **Diagnostics**: `get_host_journal` (_requires `read_host_journal` capability_).
 
 Need new packages, env vars, or other NixOS config for yourself? You can't edit your own config directly — open a PR on your `agent-configs/<name>` repo, or contact the operator directly. Config repos live at `/agents/{label}/config/` (read-only inside your container). All changes flow through operator-approved commits.