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>

internal/beads/beads_test.go

@@ -168,9 +168,9 @@ func TestFindJSONLPathDefault(t *testing.T) {
 	// Create a fake database path (no .jsonl files exist)
 	dbPath := filepath.Join(tmpDir, "test.db")
 
-	// Should return default beads.jsonl
+	// bd-6xd: Should return default issues.jsonl (canonical name)
 	result := FindJSONLPath(dbPath)
-	expected := filepath.Join(tmpDir, "beads.jsonl")
+	expected := filepath.Join(tmpDir, "issues.jsonl")
 	if result != expected {
 		t.Errorf("Expected '%s', got '%s'", expected, result)
 	}