feat(channels): add LINE Official Account channel support

Add LINE Messaging API as the 9th messaging channel using HTTP Webhook.
Supports text/image/audio messages, group chat @mention detection,
reply with quote, and loading animation.

- No external SDK required (standard library only)
- HMAC-SHA256 webhook signature verification
- Reply Token (free) with Push API fallback
- Group chat: respond only when @mentioned
- Quote original message in replies using quoteToken

Closes #146

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -9,6 +9,8 @@
 # ── Chat Channel ──────────────────────────
 # TELEGRAM_BOT_TOKEN=123456:ABC...
 # DISCORD_BOT_TOKEN=xxx
+# LINE_CHANNEL_SECRET=xxx
+# LINE_CHANNEL_ACCESS_TOKEN=xxx
 
 # ── Web Search (optional) ────────────────
 # BRAVE_SEARCH_API_KEY=BSA...