fix: restore Git sessions on link and pull

[iHildy]

Summary

Fixes issue #51.

• /sync-link first copies the synced config with a temporary session-disabled plan. It now rebuilds the session plan from that config and restores the repository's SQLite database and legacy session artifacts after the private-remote check.
• An explicit /sync-pull now reapplies Git session artifacts even when the remote branch has no new commit. It leaves unrelated local config alone and tells the user to restart OpenCode.
• A successful restore updates /sync-status's last pull time without changing the last remote update time.
• Document that this pull replaces local session artifacts with the repository snapshot.

Reproduction and verification

The regression uses a local bare Git remote and two isolated homes with a real SQLite session row. Before the fix, the linked machine had no opencode.db. After the fix, it can read the session after link and again after an up-to-date pull. The test also checks that a remote without privacy acknowledgement cannot copy the database, and that /sync-status records the successful pull.

• bun run check passed.
• bun test passed, 197 tests.
• bun run test passed, 197 tests under the pre-commit Vitest runner.
• bun run build passed.
• The full GitHub/OpenCode E2E stopped before session sync because the default model rejected API use; a retry using another external model was blocked by automatic approval review. The local Git and SQLite flow above covers the changed behavior without an external model.