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>

cmd/bd/clean.go

@@ -25,7 +25,7 @@ Files removed:
 - Git merge driver temp files (*.json[0-9], *.jsonl[0-9])
 
 Files preserved:
-- beads.jsonl (source of truth)
+- issues.jsonl (source of truth)
 - beads.db (SQLite database)
 - metadata.json
 - config.yaml