feat(i18n): add display.language for static message translation (zh/ja/de/es) (#20231)

* revert(gateway): remove stale-code self-check and auto-restart

Removes the _detect_stale_code / _trigger_stale_code_restart mechanism
introduced in #17648 and iterated in #19740. On every incoming message
the gateway compared the boot-time git HEAD SHA to the current SHA on
disk, and if they differed it would reply with

    Gateway code was updated in the background --
    restarting this gateway so your next message runs
    on the new code. Please retry in a moment.

and then kick off a graceful restart. This is unwanted behaviour:
users who run a long-lived gateway and do their own ad-hoc git
operations on the checkout end up with their chat interrupted and
the current message dropped every time HEAD moves, with no way to
opt out.

If an operator really needs the old protection against stale
sys.modules after "hermes update", the SIGKILL-survivor sweep in
hermes update (hermes_cli/main.py, also tagged #17648) already
handles the supervisor-respawn case on its own.

Removed:
  gateway/run.py:
    - _STALE_CODE_SENTINELS, _GIT_SHA_CACHE_TTL_SECS
    - _read_git_head_sha(), _compute_repo_mtime() module helpers
    - class-level _boot_wall_time / _boot_repo_mtime / _boot_git_sha /
      _stale_code_restart_triggered defaults
    - __init__ boot-snapshot block (_boot_*, _cached_current_sha*,
      _repo_root_for_staleness, _stale_code_notified)
    - _current_git_sha_cached(), _detect_stale_code(),
      _trigger_stale_code_restart() methods
    - stale-code check + user-facing restart notice at the top of
      _handle_message()
  tests/gateway/test_stale_code_self_check.py (deleted, 412 lines)

No new logic added. Zero remaining references to any removed
symbol. Gateway test suite passes the same 4589 tests it passed
before; the 3 pre-existing unrelated failures (discord free-channel,
feishu bot admission, teams typing) are unchanged by this commit.

* feat(i18n): add display.language for static message translation (zh/ja/de/es)

Adds a thin-slice i18n layer covering the highest-impact static user-facing
messages: the CLI dangerous-command approval prompt and a handful of gateway
slash-command replies (restart-drain, goal cleared, approval expired, config
read/save errors).

Out of scope (stays English): agent responses, log lines, tool outputs,
slash-command descriptions, error tracebacks.

Infrastructure:
- agent/i18n.py: catalog loader, t() helper, language resolution
  (HERMES_LANGUAGE env var > display.language config > en)
- locales/{en,zh,ja,de,es}.yaml: ~19 translated strings per language
- display.language in DEFAULT_CONFIG (hermes_cli/config.py)

Tests:
- tests/agent/test_i18n.py: 21 tests covering catalog parity, placeholder
  parity across locales, fallback behavior, env-var override, alias
  normalization, missing-key graceful degradation.

Docs:
- website/docs/user-guide/configuration.md: display.language entry plus a
  short section explaining scope so users don't expect agent responses to
  translate via this knob.

gateway/run.py

@@ -39,6 +39,7 @@ from typing import Dict, Optional, Any, List, Union
 # gateway is a long-running daemon, so its boot cost matters less than
 # preserving the established test-patch surface.
 from agent.account_usage import fetch_account_usage, render_account_usage_lines
+from agent.i18n import t
 from hermes_cli.config import cfg_get
 
 # --- Agent cache tuning ---------------------------------------------------
@@ -7377,7 +7378,7 @@ class GatewayRunner:
         if self._restart_requested or self._draining:
             count = self._running_agent_count()
             if count:
-                return f"⏳ Draining {count} active agent(s) before restart..."
+                return t("gateway.draining", count=count)
             return EphemeralReply("⏳ Gateway restart already in progress...")
 
         # Save the requester's routing info so the new gateway process can
@@ -7429,7 +7430,7 @@ class GatewayRunner:
         else:
             self.request_restart(detached=True, via_service=False)
         if active_agents:
-            return f"⏳ Draining {active_agents} active agent(s) before restart..."
+            return t("gateway.draining", count=active_agents)
         return EphemeralReply("♻ Restarting gateway. If you aren't notified within 60 seconds, restart from the console with `hermes gateway restart`.")
 
     def _is_stale_restart_redelivery(self, event: MessageEvent) -> bool:
@@ -8099,7 +8100,7 @@ class GatewayRunner:
         if lower in ("clear", "stop", "done"):
             had = mgr.has_goal()
             mgr.clear()
-            return "✓ Goal cleared." if had else "No active goal."
+            return t("gateway.goal_cleared") if had else t("gateway.no_active_goal")
 
         # Otherwise — treat the remaining text as the new goal.
         try:
@@ -9317,7 +9318,7 @@ class GatewayRunner:
         try:
             user_config: dict = _load_gateway_config()
         except Exception as e:
-            return f"⚠️ Could not read config.yaml: {e}"
+            return t("gateway.config_read_failed", error=e)
 
         effective = resolve_footer_config(user_config, platform_key)
 
@@ -9350,7 +9351,7 @@ class GatewayRunner:
             atomic_yaml_write(config_path, user_config)
         except Exception as e:
             logger.warning("Failed to save runtime_footer.enabled: %s", e)
-            return f"⚠️ Could not save config: {e}"
+            return t("gateway.config_save_failed", error=e)
 
         state = "ON" if new_state else "OFF"
         example = ""
@@ -10788,7 +10789,7 @@ class GatewayRunner:
         if not has_blocking_approval(session_key):
             if session_key in self._pending_approvals:
                 self._pending_approvals.pop(session_key)
-                return "⚠️ Approval expired (agent is no longer waiting). Ask the agent to try again."
+                return t("gateway.approval_expired")
             return "No pending command to approve."
 
         # Parse args: support "all", "all session", "all always", "session", "always"