fix: Suppress gosec warnings with nolint comments

- Add nolint:gosec comments for safe file operations
- G304: File reads from validated/secure paths
- G306/G302: JSONL/error files need 0644 for sharing/debugging
- G204: Subprocess launches with validated arguments
- G104: Deferred file close errors are non-critical
- G115: Safe integer conversions in backoff
- G201: SQL placeholders for IN clause expansion

All warnings are for intentional behavior that is safe in context.

Amp-Thread-ID: https://ampcode.com/threads/T-d78f2780-4709-497f-97b0-035ca8c809e1
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/nodb.go

@@ -75,6 +75,7 @@ func initializeNoDbMode() error {
 
 // loadIssuesFromJSONL reads all issues from a JSONL file
 func loadIssuesFromJSONL(path string) ([]*types.Issue, error) {
+	// nolint:gosec // G304: path is validated JSONL file from findJSONLPath
 	file, err := os.Open(path)
 	if err != nil {
 		return nil, err