#7100: fix(chat): add parentId to injected transcript entries

## Summary When `injectSystemEvent()` creates transcript entries via `appendAssistantTranscriptMessage()`, they were missing the `parentId` field that is required for maintaining the conversation tree structure. This caused issues during compaction in default mode. ## Changes This fix adds `getLastEntryId()` to read the last entry's id from the transcript file and uses it as `parentId` when creating new injected entries. If no previous entry exists, falls back to `sessionId`. **File affected:** - `src/gateway/server-methods/chat.ts` - Add `getLastEntryId()` and include `parentId` Fixes #7059 <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds `getLastEntryId()` to read the tail of a transcript JSONL file and uses it to set `parentId` when `appendAssistantTranscriptMessage()` injects assistant messages, aiming to preserve conversation tree structure during compaction. The change is localized to `src/gateway/server-methods/chat.ts`, affecting how injected transcript entries are written to the session transcript file. <h3>Confidence Score: 3/5</h3> - Mostly safe to merge, but there’s an unaddressed codepath that still writes injected entries without `parentId`. - The new helper is simple and low-risk, but `chat.inject` continues to bypass the fixed helper and will still emit entries missing `parentId`, which likely preserves the reported compaction issue for that pathway. - src/gateway/server-methods/chat.ts (chat.inject handler vs appendAssistantTranscriptMessage usage) <!-- 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 -->