fix: Address golangci-lint errors (errcheck, gosec) (#76)

Apply PR #76 from dannomayernotabot:

- Add golangci exclusions for internal package false positives
- Tighten file permissions (0644 -> 0600) for sensitive files
- Add ReadHeaderTimeout to HTTP server (slowloris prevention)
- Explicit error ignoring with _ = for intentional cases
- Add //nolint comments with justifications
- Spelling: cancelled -> canceled (US locale)

Co-Authored-By: dannomayernotabot <noreply@github.com>

🤖 Generated with Claude Code

internal/beads/audit.go

@@ -84,7 +84,7 @@ func (b *Beads) LogDetachAudit(entry DetachAuditEntry) error {
 	}
 
 	// Append to audit log file
-	f, err := os.OpenFile(auditPath, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0644)
+	f, err := os.OpenFile(auditPath, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0600) //nolint:gosec // G304: path is constructed internally
 	if err != nil {
 		return fmt.Errorf("opening audit log: %w", err)
 	}