Add HMAC auth, user permissions, snake_case rename

Each request is signed with HMAC-SHA256 over timestamp+body using a
per-user secret loaded from a --secrets file (never env vars or git).
Users have a canApprove list controlling who may approve queued actions.
Queue entries track submitted_by for permission checks on approve/deny.

Also renames all identifiers to snake_case throughout the codebase.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

secrets.example.json

@@ -0,0 +1,6 @@
+{
+	"users": {
+		"agent": { "secret": "change-me-agent", "canApprove": [] },
+		"user":  { "secret": "change-me-user",  "canApprove": ["agent"] }
+	}
+}