feat(config): allow placeholder text to be string or list

Allow PlaceholderConfig.Text to accept either a single string or an
array of strings, from which one is randomly selected at runtime.
This maintains backward compatibility with existing single-string configs
while enabling random placeholder selection.

Changes:
- Modify PlaceholderConfig.Text type from string to FlexibleStringSlice
- Add GetRandomText() helper method for random selection
- Update SendPlaceholder in all channels to use GetRandomText()
- Update config.example.json with array placeholder examples
- Update Matrix channel documentation

docs/channels/matrix/README.md

@@ -22,7 +22,7 @@ Add this to `config.json`:
       },
       "placeholder": {
         "enabled": true,
-        "text": "Thinking..."
+        "text": ["Thinking...", "Processing...", "Typing..."]
       },
       "reasoning_channel_id": "",
       "message_format": "richtext",
@@ -45,12 +45,19 @@ Add this to `config.json`:
 | join_on_invite       | bool     | No       | Auto-join invited rooms |
 | allow_from           | []string | No       | User whitelist (Matrix user IDs) |
 | group_trigger        | object   | No       | Group trigger strategy (`mention_only` / `prefixes`) |
-| placeholder          | object   | No       | Placeholder message config |
+| placeholder          | object   | No       | Placeholder message config (see below) |
 | reasoning_channel_id | string   | No       | Target channel for reasoning output |
 | message_format       | string   | No       | Output format: `"richtext"` (default) renders markdown as HTML; `"plain"` sends plain text only |
 | crypto_database_path | string   | No       | Path to store the crypto database (uses workspace path `~/.picoclaw/workspace` if empty) |
 | crypto_passphrase    | string   | No       | Serialization key for encrypting session keys in the database; must remain unchanged once set |
 
+### Placeholder Config
+
+| Field   | Type           | Required | Description |
+|---------|----------------|----------|-------------|
+| enabled | bool           | No       | Enable placeholder messages (default: false) |
+| text    | string/[]string | No       | Placeholder text(s). Can be a single string or array of strings. If multiple texts are provided, one is randomly selected at runtime. Default: "Thinking..." |
+
 ## 3. Currently Supported
 
 - Text message send/receive with markdown rendering (bold, italic, headers, code blocks, etc.)

docs/channels/matrix/README.zh.md

@@ -22,7 +22,7 @@
       },
       "placeholder": {
         "enabled": true,
-        "text": "Thinking... 💭"
+        "text": ["Thinking...", "Processing...", "Typing..."]
       },
       "reasoning_channel_id": "",
       "message_format": "richtext",
@@ -51,6 +51,13 @@
 | crypto_database_path | string   | 否   | 加密数据库存储路径（为空时使用工作空间路径 `~/.picoclaw/workspace`） |
 | crypto_passphrase    | string   | 否   | 加密数据库中 session key 的序列化密钥；设置后不能更改 |
 
+### 占位消息配置 (Placeholder)
+
+| 字段    | 类型            | 必填 | 说明 |
+|---------|-----------------|------|------|
+| enabled | bool            | 否   | 是否启用占位消息（默认：false） |
+| text    | string/[]string | 否   | 占位文本。可以是单个字符串或字符串数组。如果提供多个文本，运行时会随机选择一个。默认："Thinking..." |
+
 ## 3. 当前支持
 
 - 文本消息收发