Introduce slog for structured daemon logging (bd-u2sc.4)

- Replace custom daemonLogger with slog-based structured logging
- Add --log-level flag (debug, info, warn, error) with default "info"
- Add --log-json flag for JSON-formatted log output
- Update log calls to use appropriate levels (Error, Warn, Info, Debug)
- Add test helper functions (newTestLogger, newTestLoggerWithWriter)

Log output examples:
  Text: time=2025-12-23T12:30:00Z level=INFO msg="daemon started" socket=/tmp/bd.sock
  JSON: {"time":"...","level":"INFO","msg":"daemon started","socket":"/tmp/bd.sock"}

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/daemon_local_test.go

@@ -122,12 +122,8 @@ func TestCreateLocalSyncFunc(t *testing.T) {
 		t.Fatalf("Failed to create issue: %v", err)
 	}
 
-	// Create logger
-	log := daemonLogger{
-		logFunc: func(format string, args ...interface{}) {
-			t.Logf(format, args...)
-		},
-	}
+	// Create logger (test output via newTestLogger)
+	log := newTestLogger()
 
 	// Create and run local sync function
 	doSync := createLocalSyncFunc(ctx, testStore, log)
@@ -193,11 +189,7 @@ func TestCreateLocalExportFunc(t *testing.T) {
 		}
 	}
 
-	log := daemonLogger{
-		logFunc: func(format string, args ...interface{}) {
-			t.Logf(format, args...)
-		},
-	}
+	log := newTestLogger()
 
 	doExport := createLocalExportFunc(ctx, testStore, log)
 	doExport()
@@ -258,11 +250,7 @@ func TestCreateLocalAutoImportFunc(t *testing.T) {
 		t.Fatalf("Failed to write JSONL: %v", err)
 	}
 
-	log := daemonLogger{
-		logFunc: func(format string, args ...interface{}) {
-			t.Logf(format, args...)
-		},
-	}
+	log := newTestLogger()
 
 	doImport := createLocalAutoImportFunc(ctx, testStore, log)
 	doImport()
@@ -379,11 +367,7 @@ func TestLocalModeInNonGitDirectory(t *testing.T) {
 		t.Fatalf("Failed to create issue: %v", err)
 	}
 
-	log := daemonLogger{
-		logFunc: func(format string, args ...interface{}) {
-			t.Logf(format, args...)
-		},
-	}
+	log := newTestLogger()
 
 	// Run local sync (should work without git)
 	doSync := createLocalSyncFunc(ctx, testStore, log)
@@ -437,11 +421,7 @@ func TestLocalModeExportImportRoundTrip(t *testing.T) {
 	defer func() { dbPath = oldDBPath }()
 	dbPath = testDBPath
 
-	log := daemonLogger{
-		logFunc: func(format string, args ...interface{}) {
-			t.Logf(format, args...)
-		},
-	}
+	log := newTestLogger()
 
 	// Create issues
 	for i := 0; i < 5; i++ {