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/staleness.go

@@ -50,7 +50,7 @@ func ensureDatabaseFresh(ctx context.Context) error {
 			"hasn't been imported yet. This would cause you to see stale/incomplete data.\n\n"+
 			"To fix:\n"+
 			"  bd sync --import-only            # Import JSONL updates to database\n"+
-			"  bd import -i .beads/beads.jsonl  # Alternative: specify file explicitly\n\n"+
+			"  bd import -i .beads/issues.jsonl  # Alternative: specify file explicitly\n\n"+
 			"If in a sandboxed environment (e.g., Codex) where daemon can't be stopped:\n"+
 			"  bd --sandbox ready               # Use direct mode (no daemon)\n"+
 			"  bd ready --allow-stale           # Skip staleness check (use with caution)\n\n"+