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_sync_test.go

@@ -335,11 +335,7 @@ func TestExportUpdatesMetadata(t *testing.T) {
 
 	// Update metadata using the actual daemon helper function (bd-ar2.3 fix)
 	// This verifies that updateExportMetadata (used by createExportFunc and createSyncFunc) works correctly
-	mockLogger := daemonLogger{
-		logFunc: func(format string, args ...interface{}) {
-			t.Logf(format, args...)
-		},
-	}
+	mockLogger := newTestLogger()
 	updateExportMetadata(ctx, store, jsonlPath, mockLogger, "")
 
 	// Verify metadata was set (renamed from last_import_hash to jsonl_content_hash - bd-39o)
@@ -438,11 +434,7 @@ func TestUpdateExportMetadataMultiRepo(t *testing.T) {
 	}
 
 	// Create mock logger
-	mockLogger := daemonLogger{
-		logFunc: func(format string, args ...interface{}) {
-			t.Logf(format, args...)
-		},
-	}
+	mockLogger := newTestLogger()
 
 	// Update metadata for each repo with different keys (bd-ar2.2 multi-repo support)
 	updateExportMetadata(ctx, store, jsonlPath1, mockLogger, jsonlPath1)
@@ -554,11 +546,7 @@ func TestExportWithMultiRepoConfigUpdatesAllMetadata(t *testing.T) {
 
 	// Simulate multi-repo export flow (as in createExportFunc)
 	// This tests the full integration: getMultiRepoJSONLPaths -> getRepoKeyForPath -> updateExportMetadata
-	mockLogger := daemonLogger{
-		logFunc: func(format string, args ...interface{}) {
-			t.Logf(format, args...)
-		},
-	}
+	mockLogger := newTestLogger()
 
 	// Simulate multi-repo mode with stable keys
 	multiRepoPaths := []string{primaryJSONL, additionalJSONL}
@@ -676,11 +664,7 @@ func TestUpdateExportMetadataInvalidKeySuffix(t *testing.T) {
 	}
 
 	// Create mock logger
-	mockLogger := daemonLogger{
-		logFunc: func(format string, args ...interface{}) {
-			t.Logf(format, args...)
-		},
-	}
+	mockLogger := newTestLogger()
 
 	// Update metadata with keySuffix containing ':' (bd-web8: should be auto-sanitized)
 	// This simulates Windows absolute paths like "C:\Users\..."