v0.3.0 - Safe encrypted two-way sync

Adds real two-way synchronization with a persistent baseline and safe three-way merging.

• Fetches and decrypts remote changes before pushing
• Applies remote-only changes and deletions with local rollback copies
• Auto-merges non-overlapping text edits
• Stops before mutation or push on same-content conflicts
• Detects concurrent Logseq writes
• Fixes private-key headers being skipped by secret scanning
• Adds multi-device, deletion, merge, conflict, and packaging tests

Back up your age identity before upgrading. For v0.2.x migration, see the adopt-remote-baseline instructions in README.md.