chore: ignore sync state files in .beads

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.beads/.gitignore

@@ -32,6 +32,11 @@ beads.left.meta.json
 beads.right.jsonl
 beads.right.meta.json
 
+# Sync state (local-only, per-machine)
+# These files are machine-specific and should not be shared across clones
+.sync.lock
+sync_base.jsonl
+
 # NOTE: Do NOT add negation patterns (e.g., !issues.jsonl) here.
 # They would override fork protection in .git/info/exclude, allowing
 # contributors to accidentally commit upstream issue databases.