Bug: migration maps default_yolo to dead yolo field instead of default_permission_mode

[shuizhongyueming]

Description

The kimi-cli → kimi-code config migration step maps default_yolo = true to yolo = true in config.toml, but yolo is a dead field in kimi-code — no code reads it. The correct target is default_permission_mode = "yolo".

Steps to reproduce

1. Have a kimi-cli config with default_yolo = true
2. Run kimi migrate to migrate to kimi-code
3. Check ~/.kimi-code/config.toml — it contains yolo = true instead of default_permission_mode = "yolo"
4. Start kimi-code — permission mode is not yolo

Root cause

In packages/migration-legacy/src/steps/config.ts, the default_yolo branch writes migratedTop["yolo"] = true, but yolo is a dead schema field with no consumers.

Proposed fix

Change the mapping to migratedTop["default_permission_mode"] = "yolo", which is the correct config key for yolo auto-approval mode.