#19947: fix(imessage): hash-based echo detection and is_from_me early exit

## Summary Fixes #12873. When the iMessage bridge delivers the bot’s own messages back as inbound with `is_from_me` null or missing, the bot treated them as user messages and produced echo/loops. This PR fixes that. **Cause:** Previously we (1) dropped “from me” only when `is_from_me` was truthy, and (2) used a 5-second TTL text cache to suppress “recently sent bot message” echoes. The bridge sometimes sends bot messages with `is_from_me: null`, and when LLM inference exceeds 5 seconds the TTL expires so echoes get through — both mechanisms failed in those cases. **What this branch does:** Echo/duplicate handling now uses **content-hash-based detection** and **`is_from_me` early exit** together. - **Hash-based echo detection:** When the bot sends a message we store `SHA-256(scope + text)`. On inbound, if the same hash exists we treat it as a bot echo and drop it. No TTL; a FIFO cap of 200 entries and one-shot removal on match reduce false positives and bound memory. - **`is_from_me` early exit:** If the bridge reports `is_from_me: true`, we drop immediately so the same user message is not processed twice. - **Shared scope:** `echo-scope.ts` ensures delivery and inbound use the same scope key so hashes align. So: bridge-reported “from me” → early exit; when `is_from_me` is null/missing, bot echoes are caught by hash matching. **Code changes:** `SentMessageCache` is extracted into its own module and now stores/checks hashes with a bounded FIFO; `monitor-provider` uses it. In `inbound-processing` we early-return when `is_from_me` is set, then rely on the hash-based echo cache for everything else. On delivery we build scope via shared `buildIMessageEchoScope` and only store the hash in the cache. Review follow-ups: removed the stale 5-second TTL comment and the redundant pre-chunk `remember()` call. ## Test plan - [x] Unit tests for `SentMessageCache` (hash storage, echo match, FIFO eviction, one-shot removal) - [x] Unit tests for `echo-scope` (DM/group scope identical between delivery and inbound) - [x] `deliverReplies`-related tests - [ ] Manual: verify no echo loop with slow inference (e.g. local Ollama >5s)