Live sync workflow hardening and operator UX

[rgonek]

Summary

• harden live sync workflow semantics and specs for canonical paths, folder fallback, conflict handling, create previews, and attachment-aware status
• implement the workflow changes across pull, push, validation, diff, and operator prompts
• fix and extend unit/live E2E coverage, including Windows long-path handling and maintained sandbox baseline diagnostics

Verification

• make fmt-check
• make lint
• make test
• make test-e2e