Gatekeeper vs in-process boot gate: operator decision guide

[rosspeili]

Problem

README and SECURITY.md reference Gatekeeper for liveness and service boundaries; BOOT_AND_SECURITY.md compares at a high level only.

Goal

Short architecture section: when to stay on monolith gate, when to run Gatekeeper as a sidecar or replacement, migration sketch (no code required if docs-only).

Acceptance criteria

• Table or bullet comparison (threat model, ops complexity, latency).
• Linked from docs/README.md index.