Address gosec security warnings (bd-102)

- Enable gosec linter in .golangci.yml
- Tighten file permissions: 0755→0750 for directories, 0644→0600 for configs
- Git hooks remain 0700 (executable, user-only access)
- Add #nosec comments for safe cases with justifications:
  - G204: Safe subprocess launches (git show, bd daemon)
  - G304: File inclusions with controlled paths
  - G201: SQL formatting with controlled column names
  - G115: Integer conversions with controlled values

All gosec warnings resolved (20→0). All tests passing.

Amp-Thread-ID: https://ampcode.com/threads/T-d7166b9e-cbbe-4c7b-9e48-3df36b20f0d0
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/export.go

@@ -69,6 +69,7 @@ func shouldSkipExport(ctx context.Context, store storage.Storage, issue *types.I
 
 // countIssuesInJSONL counts the number of issues in a JSONL file
 func countIssuesInJSONL(path string) (int, error) {
+	// #nosec G304 - controlled path from config
 	file, err := os.Open(path)
 	if err != nil {
 		return 0, err