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/sync.go

@@ -466,7 +466,7 @@ func importFromJSONL(ctx context.Context, jsonlPath string, renameOnImport bool)
 	}
 
 	// Run import command with --resolve-collisions to automatically handle conflicts
-	cmd := exec.CommandContext(ctx, exe, args...)
+	cmd := exec.CommandContext(ctx, exe, args...) // #nosec G204 - bd import command from trusted binary
 	output, err := cmd.CombinedOutput()
 	if err != nil {
 		return fmt.Errorf("import failed: %w\n%s", err, output)