feat(sync): add remote ignore patterns

[acmore]

Summary

• add spec.sync.remoteIgnore for managed remote Syncthing .stignore patterns
• write remote .stignore before configuring Syncthing so local-only artifacts are not pulled back to remote on later starts
• keep removed spec.sync.remoteExclude rejected with guidance to use remoteIgnore
• document remoteIgnore usage for profiling or other local-only outputs

Tests

• go test ./internal/config ./internal/cli -run 'TestLoad.*Sync(RemoteIgnore|RemoteExclude)|TestWriteRemoteSTIgnoreInPod|TestBuildSTIgnoreContent|TestConfigureSyncthingPeer'
• go test ./internal/config ./internal/cli
• .venv/bin/pre-commit run --all-files --hook-stage manual okdev-gofmt