fix(zalo): fix pairing channel detection and webhook payload format

Fixes #987

Problem

1. Pairing flow failed for Zalo - The pairing system used requirePairingAdapter() which threw errors for extension channels like Zalo that aren't in the core registry.

2. Webhook returned 400 Bad Request - The webhook handler expected payloads wrapped as { ok: true, result: ZaloUpdate }, but Zalo sends updates directly as { event_name, message, ... }.

Changes

Pairing fixes:

• Replace requirePairingAdapter() with getPairingAdapter() that returns undefined for unknown channels instead of throwing
• Allow extension channels to pass their pairingAdapter directly to bypass registry lookup
• Add safeChannelKey() to sanitize channel IDs for filenames (prevents path traversal)
• Update parseChannel() in CLI to accept valid extension channel names not in core registry

Webhook fix:

• Handle both payload formats: direct { event_name, message } and wrapped { ok, result }
• Validate presence of event_name instead of ok flag

Files Changed

• extensions/zalo/src/monitor.ts - Fix webhook payload parsing, pass pairing adapter
• extensions/zalo/src/core-bridge.ts - Add pairingAdapter to upsert signature
• src/channels/plugins/pairing.ts - Accept optional pairingAdapter param
• src/pairing/pairing-store.ts - Use getPairingAdapter(), add safeChannelKey()
• src/pairing/pairing-labels.ts - Use getPairingAdapter() with fallback
• src/cli/pairing-cli.ts - Allow extension channels in parseChannel()