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/main_test.go

@@ -177,8 +177,8 @@ func TestAutoFlushJSONLContent(t *testing.T) {
 
 	dbPath = filepath.Join(tmpDir, "test.db")
 	// The actual JSONL path - findJSONLPath() will determine this
-	// but in tests it appears to be issues.jsonl in the same directory as the db
-	expectedJSONLPath := filepath.Join(tmpDir, "beads.jsonl")
+	// bd-6xd: Default is now issues.jsonl (canonical name)
+	expectedJSONLPath := filepath.Join(tmpDir, "issues.jsonl")
 
 	// Create store
 	testStore := newTestStore(t, dbPath)