#19486: fix(gateway): mirror delivered agent replies into routed target sessions

## Summary When a run delivers outbound text to a routed channel target (e.g. hook/cron with deliver=true), the message can be posted successfully but `sessions_history` for that target session may stay stale because no mirror append occurs. This change adds target-session mirroring for delivered agent replies in `deliverAgentCommandResult`. ## Root cause `deliverOutboundPayloads()` only appends transcript entries when `mirror` metadata is provided. In the affected path, delivery was executed without `mirror`. ## What changed - In `src/commands/agent/delivery.ts`: - resolve outbound session route for `(channel, to, accountId, threadId)` - pass `mirror` to `deliverOutboundPayloads` when the resolved target session differs from the source session - avoid duplicate appends when source and target session keys are the same ## Tests Added `src/commands/agent/delivery.mirror.test.ts`: 1. mirrors when routed target session differs 2. does not mirror when source and target sessions match ## Notes This is intentionally minimal and scoped to the delivery path to preserve existing behavior while restoring observability for routed deliveries. <!-- greptile_comment --> <h3>Greptile Summary</h3> This PR fixes an observability gap where agent replies delivered to routed channel targets (e.g., hook/cron routing to a Telegram chat) were successfully posted but the target session transcripts remained stale because no mirror append occurred. **Key changes:** - In `delivery.ts`: resolves the outbound session route for the delivery target and passes mirror metadata to `deliverOutboundPayloads` when the target session differs from the source session - Adds case-insensitive session key comparison to avoid duplicate appends when source and target sessions are identical - Test coverage validates both mirroring when sessions differ and skipping when they match The fix is minimal and scoped to the delivery path in `deliverAgentCommandResult`, preserving existing behavior while restoring observability for routed deliveries. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The implementation is clean, well-tested, and follows established patterns from the outbound session mirroring refactor documented in `docs/refactor/outbound-session-mirroring.md`. The change is minimal and scoped to fix a specific observability gap without affecting existing behavior. Error handling is appropriate (`.catch(() => null)` allows graceful degradation), and the case-insensitive session key comparison prevents duplicate appends. Test coverage validates both the positive case (mirroring when sessions differ) and negative case (no mirroring when sessions match). - No files require special attention _{Last reviewed commit: c9a627b} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->