fix(weixin): probe sync dir writability before choosing home

[Alix-007]

Summary

• probe the default Weixin sync cursor directory with a real temp-file write before using ~/.picoclaw
• fall back to a temp-backed picoclaw home when the default home cannot persist sync state
• add regressions for unusable HOME and existing non-writable sync directories

Testing

• go test ./pkg/channels/weixin

Fixes #1917