feat: global personal store (lore sync --global)

[franzer]

Adds a global personal store to lore sync: the user's cross-tool, cross-repo aggregate of ALL sessions, synced to a private git repo they own (managed at ~/.lore/sync), for personal multi-machine backup and search. Complements the per-repo store.

• lore sync --global setup: configure the store's remote URL + passphrase, init ~/.lore/sync, create or join the store.
• lore sync --global: push all sessions to the global store and pull others' from it.
• lore sync --global status.
• Independent tracking via a new global_synced_at column (reset alongside synced_at on any local change), so global and per-repo sync states never interfere.
• Shares the per-repo fetch/merge/orphan/carry-forward/CAS machinery via a SyncStore parameter; per-repo behavior is unchanged (its SQL and tests are byte-identical for the per-repo track).

Not wired into the pre-push hook or daemon (deferred to 29.6). New config: sync_global_remote.

Tests: global create + round-trip between machines; global pushes all sessions regardless of directory; per-repo/global track independence (each does not mark the other); both-track reset on session/child change. Full suite green, clippy -D warnings clean.