[Bug] session_index.jsonl loses newline between entries, causing session.not_found on resume

Bug Report

Summary

session_index.jsonl can end up with two JSON objects concatenated on the same line (missing newline separator), which causes kimi --session <id> to fail with [session.not_found] even when the session directory exists and is intact.

Environment

• kimi-code version: v0.27.0
• Platform: Windows x64
• Node version: v24.15.0

Steps to Reproduce

1. Create multiple sessions in the same working directory over several kimi-code launches.
2. At some point, the index write loses the newline separator between two entries.
3. Attempt to resume the earlier session by ID: kimi --session <id>.
4. Error: Failed to resume session <id>: [session.not_found]

Root Cause

session_index.jsonl had two JSON objects on the same line without a newline separator.

When the CLI reads the index line-by-line, the first line fails to parse, so the session entry becomes unreadable.

Workaround

Manually split the concatenated entry onto its own line in ~/.kimi-code/session_index.jsonl.

Notes

• The session directory is fully intact: state.json, wire.jsonl (~878KB), agents/main/ all present and valid.
• This appears to be a bug in the index append/write logic where the newline separator is occasionally lost.

Start by locating the session_index.jsonl append/write logic and the CLI code that reads the index line by line. Reproduce multiple session launches, then verify that each JSON object has its own newline and that kimi --session resumes an intact session without session.not_found.