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/import.go

@@ -39,6 +39,7 @@ Behavior:
 		// Open input
 		in := os.Stdin
 		if input != "" {
+			// #nosec G304 - user-provided file path is intentional
 			f, err := os.Open(input)
 			if err != nil {
 				fmt.Fprintf(os.Stderr, "Error opening input file: %v\n", err)