fix: Chinese path encoding and session resume on Windows

[AAAAaron]

Summary

Two fixes for Chinese/non-ASCII project paths on Windows:

1. encodePath() in src/utils/cross-platform.ts

Replace all non-ASCII characters with - to match Claude Code's actual path encoding convention. Without this, a path like D:\tx\定制绘本生成 was encoded as D--tx-定制绘本生成 but Claude Code creates sessions at D--tx-------.

2. validateSessionFile() in src/core/session-manager.ts

Stop clearing agent_session_id from the DB when the session file is not found. This allows session resume to work correctly - the ID is preserved so the SDK can attempt to locate/create the session file.

3. needsSchemaRefactor guard

Add hasIsGroup as a required guard to prevent false-positive migration triggers on already-migrated databases (which caused no such column: deleted_at errors on startup).

Test plan

• Chinese path project D:\tx\定制绘本生成 resumes existing session
• Service starts without no such column error
• Session files are found at correct ~/.claude/projects/D--tx-------/ path