#23709: fix(synology-chat): resolve Chat API user_id for reply delivery

## Summary Synology Chat outgoing webhooks use a **per-integration sequential user_id** (e.g. `1`) that differs from the **global Chat API user_id** (e.g. `4`) required by `method=chatbot`. This caused reply messages to fail silently when the IDs diverged. - Add `fetchChatUsers()` and `resolveChatUserId()` to resolve the correct Chat API user_id via the `user_list` endpoint (nickname-first matching, cached 5min) - Use resolved user_id for all `sendMessage()` calls in webhook handler and channel dispatcher - Add `Provider` field to MsgContext so the agent runner correctly identifies the message channel (was `"unknown"`, now `"synology-chat"`) - Log warnings when `user_list` API fails or when falling back to unresolved webhook user_id ## Background The Synology Chat API has two separate user_id spaces: | Context | user_id | Example | |---------|---------|---------| | Outgoing webhook (`user_id` field) | Per-integration sequential | `1` | | Chat API (`method=chatbot`, `user_ids[]`) | Global internal | `4` | The outgoing webhook's `username` field corresponds to the Chat user's `nickname`. Resolution is done by calling `method=user_list` with the bot's token and matching by nickname (primary) then username (fallback), case-insensitive. ## Test plan - [x] 68/68 unit tests pass (5 new tests for user_id resolution) - [x] 0 TypeScript errors, 0 lint warnings - [x] Tested in production: DM and group channel delivery confirmed working - [x] Verified `messageChannel=synology-chat` in agent runner logs (was `unknown`) - [x] Verified fallback behavior when user_list API is unavailable 🤖 Generated with [Claude Code](https://claude.com/claude-code) <!-- greptile_comment --> <h3>Greptile Summary</h3> Added user_id resolution to fix reply delivery failures in Synology Chat integration. The PR correctly addresses the mismatch between webhook user_id (per-integration sequential) and Chat API user_id (global internal) by fetching the user list and matching by nickname/username. The implementation includes proper caching (5min TTL), fallback behavior, and comprehensive test coverage. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The changes are well-tested (68/68 tests passing, 5 new tests), properly scoped to the Synology Chat extension, include appropriate error handling and fallback mechanisms, and follow the existing codebase patterns. The user_id resolution logic is sound with proper caching to minimize API overhead. - No files require special attention _{Last reviewed commit: ae7aeea} <!-- greptile_other_comments_section --> _{(5/5) You can turn off certain types of comments like style [here](https://app.greptile.com/review/github)!} <!-- /greptile_comment -->