Clean up database split: remove bd.jsonl and issues.jsonl duplicates

- Removed stale bd.jsonl (old data before compaction)
- Removed stale issues.jsonl (old data before compaction)
- Added config.json to track canonical JSONL filename (beads.jsonl)
- Database now has single source of truth: beads.jsonl (167 issues)
- No code changes needed - FindJSONLPath() glob correctly finds existing beads.jsonl

Amp-Thread-ID: https://ampcode.com/threads/T-f25fe193-4185-40be-b5bc-e5725a3dbf1b
Co-authored-by: Amp <amp@ampcode.com>

.beads/config.json

@@ -0,0 +1,5 @@
+{
+  "database": "beads.db",
+  "version": "0.17.7",
+  "jsonl_export": "beads.jsonl"
+}