feat(timezone): make all user/LLM-facing timestamps timezone-configurable (#129)

Add top-level `timezone` config option (default: "system") supporting
IANA names and UTC offset strings (ECMA-402 2024).

- Introduce unified `src/utils/time.ts` module with formatForLLM,
  formatLocalDate, formatLocalDateTime, startOfLocalDay, nowInstantISO
- Converge 4 scattered time helpers into the single module
- L1 extraction / auto-recall / scene-extraction / persona-generation
  prompts now emit ISO 8601 with explicit UTC offset
- Add timezone declaration to LLM system prompts
- Local-day shard boundaries and cleaner cutoff follow configured tz
- Storage instants (SQLite/TCVDB) remain UTC — no data migration needed
- 38 unit tests covering IANA, offset, DST, half-hour zones, fallback

openclaw.plugin.json

@@ -13,6 +13,11 @@
     "type": "object",
     "additionalProperties": true,
     "properties": {
+      "timezone": {
+        "type": "string",
+        "default": "system",
+        "description": "时区配置：影响所有暴露给用户/LLM 的时间戳和本地日切片。可选值：\"system\"（跟随进程系统时区）、IANA 时区名（如 \"Asia/Shanghai\"、\"Europe/Berlin\"）、UTC 偏移串（如 \"+08:00\"、\"-05:30\"）。存储时间戳始终为 UTC，此配置仅影响展示层。"
+      },
       "storeBackend": {
         "type": "string",
         "enum": ["sqlite", "tcvdb"],