bd-6xd: Standardize on issues.jsonl as canonical filename

- Change default JSONL filename from beads.jsonl to issues.jsonl
- Add bd doctor check and fix to auto-migrate legacy beads.jsonl configs
- Update FindJSONLPath to prefer issues.jsonl over beads.jsonl
- Add CheckLegacyJSONLConfig and CheckLegacyJSONLFilename checks
- Add LegacyJSONLConfig fix to rename files and update config
- Update .gitattributes to reference issues.jsonl
- Fix tests to expect new canonical filename
- Add bd-6xd to v0.25.1 release notes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.beads/deletions.jsonl

@@ -49,3 +49,4 @@
 {"id":"bd-z3s3","ts":"2025-11-25T22:40:41.19109Z","by":"stevey","reason":"cleanup"}
 {"id":"bd-5a90","ts":"2025-11-25T22:40:41.19621Z","by":"stevey","reason":"cleanup"}
 {"id":"bd-5b6e","ts":"2025-11-25T22:40:41.201204Z","by":"stevey","reason":"cleanup"}
+{"id":"bd-98c4e1fa.1","ts":"2025-11-27T04:17:14.657783Z","by":"git-history-backfill","reason":"recovered from git history (pruned from manifest)"}