#16893: fix(threads): inject thread starter context on every turn, not just new sessions

## Summary Fixes #4927 The `ThreadStarterBody` injection in `get-reply-run.ts` was gated by `isNewSession`, which means follow-up messages in a thread lost the parent message context. This caused the bot to have no knowledge of what started the thread after the first reply. **Root cause:** When the first thread reply creates the session, `isNewSession = true` and the thread starter is injected. But subsequent messages find the session already exists, so `isNewSession = false` and the thread starter body is silently dropped — even though the Slack message handler correctly fetches it for every thread message. **Fix:** Remove the `isNewSession` gate from the `threadStarterBody` branch. The thread starter is a small context note (just the parent message text) that should be available on every turn. The `threadHistoryBody` (full thread history dump) retains the `isNewSession` gate since it's large and only needed once. This affects all channels with thread support (Slack, Discord) that use `inheritParent: true`. ## Test plan - [x] Existing thread tests pass (`pnpm test -- src/auto-reply/reply/get-reply-run.media-only.test.ts src/slack/monitor.tool-result.threads-top-level-replies-replytomode-is-all.test.ts`) - [x] Verified thread starter is always populated by Slack/Discord prepare handlers for thread messages - [x] `threadHistoryBody` still gated by `isNewSession` (no change in behavior for large history dumps) 🤖 Generated with [Claude Code](https://claude.com/claude-code) <!-- greptile_comment --> <h3>Greptile Summary</h3> Removed the `isNewSession` gate from `threadStarterBody` injection so that thread parent context is available on every turn, not just the first reply. Previously, follow-up messages in threads lost context of the parent message that started the thread. The `threadHistoryBody` (full thread history) correctly retains the `isNewSession` gate since it's large and only needed once. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The change is a minimal, well-targeted fix that removes an incorrect conditional gate. The logic is sound: `threadStarterBody` is a small context note (just the parent message text) that should be available on every turn in a thread, while `threadHistoryBody` (full history dump) correctly remains gated by `isNewSession`. The test coverage is good with a new test specifically validating the fix, and the change aligns with how Slack/Discord handlers populate thread starter context for every thread message. - No files require special attention _{Last reviewed commit: b92b28a} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->