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.

website/docs/user-guide/configuration.md

@@ -1167,6 +1167,20 @@ display:
   show_cost: false        # Show estimated $ cost in the CLI status bar
   tool_preview_length: 0  # Max chars for tool call previews (0 = no limit, show full paths/commands)
   runtime_metadata_footer: false  # Gateway: append a runtime-context footer to final replies
+  language: en            # UI language for static messages (approval prompts, some gateway replies). en | zh | ja | de | es
+```
+
+### UI language for static messages
+
+The `display.language` setting translates a small set of static user-facing messages — the CLI approval prompt, a handful of gateway slash-command replies (e.g. restart-drain notices, "approval expired", "goal cleared"). It does **not** translate agent responses, log lines, tool output, error tracebacks, or slash-command descriptions — those stay in English. If you want the agent itself to reply in another language, just tell it in your prompt or system message.
+
+Supported values: `en` (default), `zh` (Simplified Chinese), `ja` (Japanese), `de` (German), `es` (Spanish). Unknown values fall back to English.
+
+You can also set this per-session with the `HERMES_LANGUAGE` env var, which overrides the config value.
+
+```yaml
+display:
+  language: zh   # CLI approval prompts appear in Chinese
 ```
 
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