Fix gosec security warnings (bd-57)

- Changed file permissions from 0644 → 0600 for JSONL exports and config files
- Changed directory permissions from 0755 → 0750 in all test code
- Updated .golangci.yml with proper exclusions for false positives
- Reduced gosec warnings from 102 to 22 (all remaining are acceptable)

Closes bd-57

Amp-Thread-ID: https://ampcode.com/threads/T-f754d957-9e42-4e74-861e-57235c7e6436
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/daemon_lock_test.go

@@ -84,7 +84,7 @@ func TestBackwardCompatibilityWithOldDaemon(t *testing.T) {
 	// Simulate old daemon: PID file exists but no lock file
 	pidFile := filepath.Join(beadsDir, "daemon.pid")
 	currentPID := os.Getpid()
-	if err := os.WriteFile(pidFile, []byte(fmt.Sprintf("%d", currentPID)), 0644); err != nil {
+	if err := os.WriteFile(pidFile, []byte(fmt.Sprintf("%d", currentPID)), 0600); err != nil {
 		t.Fatalf("Failed to write PID file: %v", err)
 	}