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>

internal/types/lock_check.go

@@ -20,7 +20,7 @@ func ShouldSkipDatabase(beadsDir string) (skip bool, holder string, err error) {
 	lockPath := filepath.Join(beadsDir, ".exclusive-lock")
 
 	// Check if lock file exists
-	data, err := os.ReadFile(lockPath)
+	data, err := os.ReadFile(lockPath) // #nosec G304 - controlled path from config
 	if err != nil {
 		if os.IsNotExist(err) {
 			// No lock file, proceed with database