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/comments.go

@@ -114,7 +114,7 @@ Examples:
 		commentText, _ := cmd.Flags().GetString("file")
 		if commentText != "" {
 			// Read from file
-			data, err := os.ReadFile(commentText)
+			data, err := os.ReadFile(commentText) // #nosec G304 - user-provided file path is intentional
 			if err != nil {
 				fmt.Fprintf(os.Stderr, "Error reading file: %v\n", err)
 				os.Exit(1)