fix: logger Error serialization, config validation on startup, dedup cleanup, sk-* secret masking

[Fengsh0923]

Summary

Four targeted bug fixes found during a code review:

1. logger.ts — Error objects serialize to {} (loses stack trace)

Before:
```ts
args.map((a) => (typeof a === 'string' ? a : JSON.stringify(a)))
```
JSON.stringify(new Error('boom')) returns '{}' — Error properties are non-enumerable. Stack traces are lost, making daemon errors very hard to debug.

After:
```ts
args.map((a) => {
if (typeof a === 'string') return a;
if (a instanceof Error) return a.stack ?? a.message;
return JSON.stringify(a);
})
```

2. logger.ts — sk-* API keys not masked in logs

OpenAI / Anthropic / third-party API keys (format: sk-<20+ chars>) were not covered by MASK_PATTERNS. Added:
```ts
/sk-[A-Za-z0-9]{20,}/g,
```

3. config.ts — No validation of required fields per channel

The daemon started silently even when required credentials were missing (e.g. CTI_FEISHU_APP_ID absent while feishu was in CTI_ENABLED_CHANNELS). The error only surfaced on the first user message with a cryptic SDK error.

Added validateConfig(config: Config): ConfigValidationError[] that checks:

• At least one channel is enabled
• Telegram: token + (chatId or allowedUsers)
• Feishu: appId + appSecret
• Discord: botToken + (allowedUsers or allowedChannels)
• QQ: appId + appSecret

4. main.ts — cleanupExpiredDedup() existed but was never called

store.cleanupExpiredDedup() was implemented in JsonFileStore but never invoked, causing the dedup Map (and its backing dedup.json file) to grow without bound over time.

Added a 10-minute unref()'d interval so cleanup runs automatically without blocking shutdown.

main.ts also now calls validateConfig() at startup and exits with clear per-field error messages if config is invalid.

Test plan

• Start daemon with a missing required field (e.g. remove CTI_FEISHU_APP_ID) — should exit immediately with a clear error message listing the missing field
• Trigger an uncaught Error in the daemon — stack trace should now appear in bridge.log
• Verify sk-xxx values are masked in log output
• After many messages, confirm dedup.json file size stays bounded