#23229: fix(subagent-announce): gate queued delivery on channel deliverability and remove method:send bypass

## Summary - **`sendAnnounce` (queued path)**: Added `channelIsDeliverable` guard so channel hints and `deliver:true` are only set when the origin channel is actually registered and routable. Without this, an unroutable channel causes a 15s gateway timeout × 3 retries, then fallback to stale session routes (typically WhatsApp), producing `Error: Delivering to WhatsApp requires target <E.164|group JID>`. - **`sendSubagentAnnounceDirectly`**: Removed the `method:send` fast path that pushed raw subagent output directly to the user, bypassing the requester agent entirely. All completion announces now go through `method:agent` so the requester agent processes the result and formulates a reply in its own voice. The existing `directOrigin` hints (channel/to/accountId/threadId) on the `method:agent` call are preserved for explicit delivery routing. ## Related Companion to #23030 (restore `isInternalMessageChannel` guard in `resolveAnnounceOrigin`). Both PRs address subagent announce delivery failures on plugin channels like Gmail. ## Test plan - [ ] Spawn a subagent from a Gmail thread — completion announce delivers via Gmail, not WhatsApp - [ ] Spawn a subagent from WhatsApp — completion announce still delivers via WhatsApp (no regression) - [ ] Spawn a subagent from webchat — completion announce delivers via webchat (internal channel handling unaffected) 🤖 Generated with [Claude Code](https://claude.com/claude-code) <!-- greptile_comment --> <h3>Greptile Summary</h3> This PR fixes subagent completion announce delivery failures on plugin channels (e.g., Gmail) by adding two defensive guards: **Change 1 - `sendAnnounce` (queued path):** Added `channelIsDeliverable` check to gate channel delivery hints. Previously, the code only checked `requesterIsSubagent` before setting channel params, which meant unroutable channels (like Gmail when not registered) would still trigger delivery attempts, leading to 15s gateway timeouts × 3 retries, then fallback to stale WhatsApp routes. **Change 2 - `sendSubagentAnnounceDirectly`:** Removed the `method:send` fast path that pushed raw subagent output directly to users. All completion announces now route through `method:agent` so the requester agent can process results and formulate replies in its own voice, while preserving `directOrigin` hints for explicit routing. Both changes improve reliability for plugin channels and maintain consistency in the announce delivery flow. <h3>Confidence Score: 4/5</h3> - This PR is safe to merge with minor risk - logic changes are well-scoped and defensive - The changes add defensive guards to prevent delivery failures on unroutable channels and simplify the announce flow by removing the `method:send` bypass. The logic is sound and well-tested (existing tests pass based on test file analysis). However, the first change modifies conditional logic in a subtle way that could affect edge cases not covered by tests. - No files require special attention - the single changed file has focused, defensive improvements _{Last reviewed commit: aef086a} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->