#5514: Slack: fix threadId ignored on read + thread_broadcast dropped

## Summary - **Bug 1 (#2454):** The Slack extension's `action=read` handler was not passing `threadId` through to `handleSlackAction`. Added the missing `threadId` param in `extensions/slack/src/channel.ts`. - **Bug 2 (#4351):** Messages with `subtype === "thread_broadcast"` (thread replies sent with "Also send to channel") were dropped because the handler returned early after logging a system event. Removed the early `return` in `src/slack/monitor/events/messages.ts` so these messages fall through to `handleSlackMessage`. Closes #2454, Closes #4351 ## Test plan - [x] `pnpm build` passes - [x] `pnpm vitest run src/slack/ extensions/slack/` — all 126 tests pass - [ ] Manual: send a thread broadcast in Slack and verify it's processed - [ ] Manual: read messages with a `threadId` param via the extension and verify thread messages are returned 🤖 Generated with [Claude Code](https://claude.com/claude-code) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR fixes two Slack message handling regressions: - In the Slack extension channel plugin, the `action=read` handler now forwards `threadId` into the `readMessages` tool payload so threaded reads can be scoped correctly. - In the core Slack monitor message event handler, `thread_broadcast` messages no longer return early after logging a system event; they now continue through the shared `handleSlackMessage(...)` path, preventing these user-visible messages from being dropped. Both changes are small and localized, and they align the extension adapter behavior with the existing core Slack actions adapter implementation. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - Changes are narrowly scoped (one added param passthrough and one removed early return) and they restore intended Slack behavior without altering interfaces or control flow beyond the specific bug conditions; the extension change matches existing core adapter behavior and there are no new edge cases apparent in the touched code paths. - 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 -->