feat(channel): add mention_only option for Telegram groups

Adds mention_only config option to Telegram channel, allowing the bot
to only respond to messages that @-mention the bot in group chats.
Direct messages are always processed regardless of this setting.

Behavior:
- When mention_only = true: Bot only responds to group messages containing @botname
- When mention_only = false (default): Bot responds to all allowed messages
- DM/private chats always work regardless of mention_only setting

Implementation:
- Fetch and cache bot username from Telegram API on startup
- Check for @botname mention in group messages
- Strip mention from message content before processing

Config example:
[channels.telegram]
bot_token = "your_token"
mention_only = true

Changes:
- src/config/schema.rs: Add mention_only to TelegramConfig
- src/channels/telegram.rs: Implement mention_only logic + 6 new tests
- src/channels/mod.rs: Update factory calls
- src/cron/scheduler.rs: Update constructor call
- src/onboard/wizard.rs: Update wizard config
- src/daemon/mod.rs: Update test config
- src/integrations/registry.rs: Update test config
- TESTING_TELEGRAM.md: Add mention_only test section
- CHANGELOG.md: Document feature

Risk: medium
Backward compatible: Yes (default: false)

CHANGELOG.md

@@ -18,6 +18,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   value if the input used the legacy `enc:` format
 - `SecretStore::needs_migration()` — Check if a value uses the legacy `enc:` format
 - `SecretStore::is_secure_encrypted()` — Check if a value uses the secure `enc2:` format
+- **Telegram mention_only mode** — New config option `mention_only` for Telegram channel.
+  When enabled, bot only responds to messages that @-mention the bot in group chats.
+  Direct messages always work regardless of this setting. Default: `false`.
 
 ### Deprecated
 - `enc:` prefix for encrypted secrets — Use `enc2:` (ChaCha20-Poly1305) instead.