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

pkg/config/config.go

@@ -3,6 +3,7 @@ package config
 import (
 	"encoding/json"
 	"fmt"
+	"math/rand"
 	"os"
 	"path/filepath"
 	"strings"
@@ -381,8 +382,20 @@ type TypingConfig struct {
 
 // PlaceholderConfig controls placeholder message behavior (Phase 10).
 type PlaceholderConfig struct {
-	Enabled bool   `json:"enabled"`
-	Text    string `json:"text,omitempty"`
+	Enabled bool               `json:"enabled"`
+	Text    FlexibleStringSlice `json:"text,omitempty"`
+}
+
+// GetRandomText returns a random placeholder text, or default if none set.
+func (p *PlaceholderConfig) GetRandomText() string {
+	if len(p.Text) == 0 {
+		return "Thinking..."
+	}
+	if len(p.Text) == 1 {
+		return p.Text[0]
+	}
+	idx := rand.Intn(len(p.Text))
+	return p.Text[idx]
 }
 
 type StreamingConfig struct {

pkg/config/defaults.go

@@ -62,7 +62,7 @@ func DefaultConfig() *Config {
 				Typing:    TypingConfig{Enabled: true},
 				Placeholder: PlaceholderConfig{
 					Enabled: true,
-					Text:    "Thinking... 💭",
+					Text:    FlexibleStringSlice{"Thinking... 💭"},
 				},
 				Streaming:     StreamingConfig{Enabled: true, ThrottleSeconds: 3, MinGrowthChars: 200},
 				UseMarkdownV2: false,
@@ -111,7 +111,7 @@ func DefaultConfig() *Config {
 				},
 				Placeholder: PlaceholderConfig{
 					Enabled: true,
-					Text:    "Thinking... 💭",
+					Text:    FlexibleStringSlice{"Thinking... 💭"},
 				},
 				CryptoDatabasePath: "",
 				CryptoPassphrase:   "",