feat: scaffold deployment-policies monorepo

OPA/Rego deployment gateway policies: 5 rules (valid-environment,
require-approvals, no-self-approval, block-weekends, freeze-window),
a deploy-gate combining them, JSON schemas, scenario-driven tests,
Gitea Actions CI (verify + publish to generic registry), changesets
versioning, Makefile and scripts.

policies/rules/valid-environment/policy.rego

@@ -0,0 +1,19 @@
+# METADATA
+# schemas:
+#   - input: schema["input"]
+#   - data: schema["data"]
+
+package rules.valid_environment
+
+import rego.v1
+
+default allow := false
+
+allow if {
+	count(violations) == 0
+}
+
+violations contains {"code": "unknown_environment", "message": sprintf("environment %q is not configured", [input.deployment.environment])} if {
+	input.deployment
+	not data.deploygate.config[input.deployment.environment]
+}