#23030: fix(subagents): restore isInternalMessageChannel guard in resolveAnnounceOrigin

## Summary Restores the narrower `isInternalMessageChannel()` guard from PR #22223 (fe57bea08) that was inadvertently reverted by f555835b0. The original `!isDeliverableMessageChannel()` check strips the requester channel from the announce origin whenever the channel is not in the registered deliverable set. This is overly broad: it discards legitimate requester origins for plugin channels whose adapter ID differs from their plugin ID (e.g. `"gmail"` vs `"openclaw-gmail"`), causing the announce system to fall back to stale session routes. `isInternalMessageChannel()` limits stripping to explicitly internal channels (webchat only), preserving requester origin context for all external channels regardless of their position in the deliverable registry. ## Note This is a behaviour change for channels not yet in the deliverable list. Acceptable as a no-op if upstream prefers the broader check — the immediate Gmail delivery issue is fixed separately in #23229. ## Related - #23229 — companion fix: gate `sendAnnounce` delivery hints on channel deliverability, remove `method:send` bypass in `sendSubagentAnnounceDirectly` - #22223 — original fix (fe57bea08) that introduced `isInternalMessageChannel` - f555835b0 — commit that inadvertently reverted #22223 ## Test plan - [ ] Spawn a subagent from a Gmail thread — announce origin channel is preserved, not stripped - [ ] Spawn a subagent from webchat — webchat still stripped as internal (no regression) 🤖 Generated with [Claude Code](https://claude.com/claude-code)