#12369: fix: register unhandled rejection handler for Slack monitor

## Summary - Slack monitor crashes the entire gateway process when a transient network error (ECONNRESET, ETIMEDOUT, etc.) escapes Slack Bolt's internal handling as an unhandled promise rejection - Telegram (`src/telegram/monitor.ts:97`) and WhatsApp (`src/web/auto-reply/monitor.ts:229`) monitors already register `registerUnhandledRejectionHandler()` to suppress these transient errors gracefully - This adds the same pattern to the Slack monitor: transient network errors are logged and suppressed instead of crashing ## Test plan - [x] Follows established pattern from Telegram/WhatsApp monitors - [x] Handler registered before `app.start()`, cleaned up in `finally` block - [x] Only suppresses errors matching `isTransientNetworkError()` — all other errors propagate normally - [x] TypeScript compiles without errors Closes #12354 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com> <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This change adds an `unhandledRejection` suppression hook to the Slack monitor provider so transient network failures (e.g., `ECONNRESET`, `ETIMEDOUT`) that escape Slack Bolt don’t crash the whole gateway process. It follows the existing cross-channel pattern by registering a handler before `app.start()` and unregistering it in the `finally` cleanup path, while allowing non-transient errors to continue propagating through the global unhandled-rejection handler. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - The change is small, localized to the Slack monitor startup/teardown path, and matches the existing unhandled-rejection suppression pattern already used by other monitors. The handler only suppresses well-defined transient network errors and is properly unregistered in the `finally` block; the global `installUnhandledRejectionHandler()` is already installed by the main entrypoint, so this hook will be effective at runtime. - No files require special attention <!-- greptile_other_comments_section --> **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->