feat: /goal — persistent cross-turn goals (Ralph loop) (#18262)

Add a standing-goal slash command that keeps Hermes working toward a
user-stated objective across turns until it is achieved, paused, or
the turn budget runs out. Our take on the Ralph loop — cf. Codex CLI
0.128.0's /goal.

After each turn, a lightweight auxiliary-model judge call asks 'is
this goal satisfied by the assistant's last response?'. If not, and
we're under the turn budget (default 20), Hermes feeds a continuation
prompt back into the same session as a normal user message. Any real
user message preempts the continuation loop automatically.

Judge failures fail OPEN (continue) so a flaky judge never wedges
progress — the turn budget is the real backstop.

### Commands

- `/goal <text>`    — set a standing goal (kicks off the first turn)
- `/goal` or `/goal status` — show current state
- `/goal pause`    — pause the continuation loop
- `/goal resume`   — resume (resets turn counter)
- `/goal clear`    — drop the goal

Works on both CLI and gateway platforms via the central CommandDef
registry.

### Design invariants preserved

- **Prompt cache**: continuation prompts are regular user-role
  messages appended to history. No system-prompt mutation, no toolset
  swap.
- **Role alternation**: continuation is a user turn, never injected
  mid-tool-loop.
- **Session persistence**: goal state lives in SessionDB.state_meta
  keyed by `goal:<session_id>`, so `/resume` picks it up.
- **Mid-run safety**: on the gateway, `/goal status|pause|clear` are
  allowed mid-run (control-plane only); setting a new goal requires
  `/stop` first so we don't race a second continuation prompt against
  the current turn.

### Files

- `hermes_cli/goals.py` (new, 380 lines) — GoalManager + judge + state
- `hermes_cli/commands.py` — CommandDef entry
- `hermes_cli/config.py` — `goals.max_turns` default
- `hermes_cli/web_server.py` — dashboard category merge
- `cli.py` — /goal handler + post-turn continuation hook in
  process_loop
- `gateway/run.py` — /goal handler + post-turn continuation hook
  wrapping _handle_message_with_agent
- `tests/hermes_cli/test_goals.py` (new, 26 tests) — judge parsing,
  fail-open semantics, lifecycle, persistence, budget exhaustion
- `website/docs/reference/slash-commands.md` — docs entry

hermes_cli/web_server.py

@@ -345,6 +345,7 @@ _CATEGORY_MERGE: Dict[str, str] = {
     "dashboard": "display",
     "code_execution": "agent",
     "prompt_caching": "agent",
+    "goals": "agent",
     # Only `telegram.reactions` currently lives under telegram — fold it in
     # with the other messaging-platform config (discord) so it isn't an
     # orphan tab of one field.