docs: add git-ref sync to README

[franzer]

Factual README update for git-ref sync (no repositioning).

• Adds lore sync setup / lore sync to Quick Start.
• Adds a "Git-ref Sync" row to Key Features and marks the daemon "(optional)".
• Adds a Sync section: per-repo store, global store, the pre-push hook, team sharing via passphrase, serverless; links to the sync guide.
• Softens the storage note to reflect opt-in, end-to-end-encrypted sync.