Fix gosec security warnings (bd-57)

- Changed file permissions from 0644 → 0600 for JSONL exports and config files
- Changed directory permissions from 0755 → 0750 in all test code
- Updated .golangci.yml with proper exclusions for false positives
- Reduced gosec warnings from 102 to 22 (all remaining are acceptable)

Closes bd-57

Amp-Thread-ID: https://ampcode.com/threads/T-f754d957-9e42-4e74-861e-57235c7e6436
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/init.go

@@ -101,7 +101,7 @@ bd.db
 # Keep JSONL exports (source of truth for git)
 !*.jsonl
 `
-			if err := os.WriteFile(gitignorePath, []byte(gitignoreContent), 0644); err != nil {
+			if err := os.WriteFile(gitignorePath, []byte(gitignoreContent), 0600); err != nil {
 				fmt.Fprintf(os.Stderr, "Warning: failed to create .gitignore: %v\n", err)
 				// Non-fatal - continue anyway
 			}