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_test.go

@@ -49,7 +49,8 @@ func TestFindJSONLPath(t *testing.T) {
 	}
 
 	jsonlPath := beads.FindJSONLPath(dbPath)
-	expectedPath := filepath.Join(tmpDir, ".beads", "beads.jsonl")
+	// bd-6xd: Default is now issues.jsonl (canonical name)
+	expectedPath := filepath.Join(tmpDir, ".beads", "issues.jsonl")
 
 	if jsonlPath != expectedPath {
 		t.Errorf("FindJSONLPath returned %s, expected %s", jsonlPath, expectedPath)